It is being speculated in the press that they flew through a flock of Canada Geese. "Honkers" as they are known, are very large birds, with adult males reaching 7-14 pounds in weight and females 6-12 pounds. Just one of these birds would be large enough to take out an engine of the A320, and it is possible that they injested more than one into each.
It is a testament to the skills of the flight crew that they manages to dead-stick a large, Transport Category airplane to a ditching. 
I do not think that is something that is practiced in the simulators, but I could be wrong. What I am curious about is whether there was a backup hydraulic system operated by the tail mounted APU that would drive the hydraulic flight controls, slats, and flaps. I do know that the APU is normally on for takeoff so there would have been no loss of electrical power or fly-by-wire control.
Can anyone here provide us with a little more information into the A320 systems please?
RC
I can't give you A320 systems right now, but I should be able to in a while, I know a few people who fly them.
I'll note that I would be surprised if the APU was left on for takeoff on such a large aircraft. On the CRJ, which has very similiar systems on a smaller scale, we only use the APU on takeoff in very specific circumstances. Two reasons are heat and cooling on the ground since the engines at low rpm don't produce much bleed air (not usually a problem with the large birds). The third is any time we takeoff with our ice protection on, because the heated wings take so much bleed air. But that is due to the limitations of our engines' power output. On larger aircraft, the engines have no problem providing the bleed air to run both the ice protection systems and the packs (pneumatic air conditioning kits) at the same time. I don't recall being on any large transport category aircraft where the APU was left on for takeoff. I don't fly the A320, but I sit in the cockpit on A320s, 737s, 757s and MD-88s almost as much as I fly myself (one month I spent more time in the jumpseat than at the controls). There are a couple reasons to leave an APU on otherwise, but they are due to deferal or other MX situations.
Since it was a clear day in the area, I they wouldn't have even needed ice protection so I'd put the chances of the APU operating at one in 100,000. However, depending on how much time they had from the first engine failure, they would have started the APU when they lost the first engine to provide backup to the remaining engine's generator and ensure adequate electrical power.
Even on the CRJ, we turn off our APU in almost all cases passing 3000' AGL for our climb checks. I believe the NWA A320s I've ridden on have had their climb checks completed at a lower altitude, so even if the APU was on for takeoff (once again, I doubt it), it would probobly have been shut down by the time this incident happened if the time frames I've seen were accurate.
Rather than need the APU for hydraulics, the RAT (Ram Air Turbine), sometimes known as an ADG (Air Driven Generator) automatically deploys upon loss of power from both engine driven generators. It gives minimal hydraulics and electrical systems. There is a spool up time, but it's in the seconds.
There should be electrical hydraulic pumps that the APU can run as well. We can only run one hydraulic pump (our system #3 pump) if only the APU is operating, but we can run it. That is the same pump that the ADG will run if it is deployed.
Also, it's perfectly normal for an APU to actually be deferred and inoperative for a regular flight. I almost did that last night actually. They didn't have ground heat at the destination and we were staying there 5 hours so we didn't because we wouldn't be able to heat the plane in the morning, but that's the only reason. The primary purpose of the APU is to start the engines, everything else is just a bonus becuase it's there.
