It's funny... only a few hours ago I posted elsewhere, agreeing that the most energetic finger pointers could probably do better things with their time, because the focus right now should really be on getting people to safety, and only afterwards working out how to prevent it from happening again.
After having decided to poke my nose into what was actually being said, I followed a thread of accounts of FEMA, the National Guard, the border patrol, and President Bush, himself, personally, not only not helping, but actually getting in the way of rescue attempts, I am fuming, and have changed my mind. Someone please document and nail the bastards, and yell about it until someone listens. It's killing people now.
I'll start the trail several days ago, along the Canadian border. Canada's specialized urban search and rescue team (related to DART?) was the first non-US team to be requested... but then they weren't allowed to cross the border (hat tip: Majikthise). This eventually did get corrected, and they were allowed through, though the situation was so bad when they arrived that they had to delay operations.
Next, we find out that the US Northern Command in Colorado, responsible for deployment of military personnel and supplies, was actually ready to go before the storm hit... but could not move out because no authorization had come from the President (they eventually deployed after Bush interrupted his vacation on August 31, several days late, and will only be arriving this weekend). The White House at one point claimed that they couldn't come in until they were asked for help by the state, which would be understandable, except that Louisiana begged them for help on Sunday, August 28. (As a side note, there is a pictorial timeline of what Bush was doing between August 28 and August 31. None of it looks particularly helpful, especially in light of what's in the next paragraph.)
Then, when Bush actually does make it to New Orleans, the White house grounds the helecopters transporting food while he is there. This is a day after a hundred people who were rescued from their rooftops died anyway at the Chalmette shelter for lack of supplies. Why were the helicopters grounded? It's not clear; I suppose it could be for security considerations (in which case, he shouldn't have actually been there physically if he knew it would disrupt operations), but it may have been so he could have a couple sitting in the background for his photo op. He apparently did the same for a photo op on the 17th Street levee, with equipment that was brought there for the cameras gone the next day, something that Louisiana Senator Landrieu complained bitterly of in a statement requesting the appointment a cabinet-level official to oversee Hurricane Katrina relief and recovery efforts within 24 hours. That statement also disclosed that equipment and assistance made available both by the U.S. Forest Service to put out fires and by Amtrak to evacuate citizens faster than any bus could manage were both ignored by FEMA, along with other offers of assistance.
Update: The food distribution photo op that Bush was involved with was apparently also staged, with the food distribution points being torn down as soon the cameras stopped rolling.
Some of those offers came from a flotilla of expert boaters, organized by La. State Senator, Nick Gautreaux from Vermilion Parish. 500 boats, 1000 boaters, with boats specially designed for handicapped or wheelchair access, were turned back by FEMA, which told them that they weren't needed. That night, CNN had an interview with a nurse trapped inside Charity Hospital, who said that there were a thousand people trapped inside the hospital with no food and no medical supplies, and that only three had been rescued since the hurricane hit. That flotilla could have gotten them all out.
Mayor Daley of Chicago, Illinois also offered significant aid last Sunday:
Daley said the city offered 36 members of the firefighters' technical rescue teams, eight emergency medical technicians, search-and-rescue equipment, more than 100 police officers as well as police vehicles and two boats, 29 clinical and 117 non-clinical health workers, a mobile clinic and eight trained personnel, 140 Streets and Sanitation workers and 29 trucks, plus other supplies. City personnel are willing to operate self-sufficiently and would not depend on local authorities for food, water, shelter and other supplies, he said.
The only thing FEMA would allow them to send was a single tank truck on Friday.
And now it turns out that FEMA wouldn't let the Red Cross into New Orleans for fear that if they bring food and medical supplies to the people trapped there, the residents wouldn't want to leave... despite the fact that the Superdome was only finally evacuated about an hour ago as I write this, and things got very ugly there before they finally did. The guards themselves were a trigger-happy threat, gunning down one 16-year-old boy in the street, and shooting dead another man who ran to them to get help for a woman being raped.
I wonder if some of those were from units brought back prematurely from Iraq. Maybe it's just as well that Bush refused to bring back more. Maybe it was overcompensation for the fact that someone noticed that for the first three days, they were playing cards while the police and firefighters fought a losing battle to get the city under control. New Orleans wasn't the only place abandoned by the National Guard that was still there: Biloxi, Mississippi witnessed them playing basketball and performing calisthenics rather than help clean up and provide aid.
If you hear about further obstruction of assistance to remaining survivors, pass the word and raise a fuss.

Iraq troops
You mentioned that "[You] wonder if some of those were from units brought back prematurely from Iraq." This is very possible, as several National Guard troops were brought straight to New Orleans from Iraq. There was a story on CNN about how they had to be reminded by their commanding general that they were no longer in Iraq, and then were ordered to point their guns down.
Record it all and archive it!
If you haven't yet discovered Air America Radio, I'm recommending it to any and everyone. Afternoon show host Randi Rhodes has compiled a great collection of information on her website including timelines and copies of the letters that were sent in to request/enact a state of emergency. Check it out at http://www.therandirhodesshow.com/live/
Like Zed says, document it, archive it! We've got to get this information and documentation so that these double-speaking bureaucrats can and will be held accountable not only for their inaction and ineptitude but their IMMEDIATE and persistent lies about everything. Meanwhile KBR, a subsudary of Haliburton, has been granted contracts in the disaster area while domestic and international help are steadily being turned away.
I know that CYA (aka covering your ass) has practically become an American pastime, but I hope that this administration's level of spin while tens of thousands of lives hang in the balance will finally serve as a huge, overdue wake up call! And I mean all of us, because if it weren't so acceptable to us in the first place, this Administration wouldn't have gotten away with it for as long as they have. IT HAS TO STOP - and we've ALL got to do our part to make it happen.
How much more proof do we need that this man needs to be impeached! Let's hold this president and his administration accountable with the truth and it will set us all free (or at least keep us emboldened to keep fighting)!