Via Rob Staton of Seahawks Draft Blog.
My amateur take: Mallett's problem isn't his footwork or ability to throw on the run. What hurts Mallett is his lack of escapability. His extremely limited ability to evade a defender keeps him from being able to find space to set his feet. It's not that he can't throw on the run, he can, it's that he's not quick or agile enough to try. He throws off his back foot not because he has poor fundamentals but because he's not able to buy himself time to do anything else.
Fortunately for Mallett he has what might be the strongest arm in any level of football. He can get away with throwing off his back foot because he's still able to get more oomph on the ball than some NFL QBs get with a perfect pocket. Thats only part of the equation of course, but his accuracy doesn't suffer too terribly. Watching the video, more often than not Mallett's innacurate throws under pressure are bounced well before the receiver or launched over the receiver's head towards the side line. To me they seem to be throw aways, he rarely makes a dangerously innacurate throw. The pick at 6:09 is an example of this gone awry, the throw is tossed well over the receiver's head but not well-over enough and the Bama safety is able to make a great play on the ball.
Theres no way around it, Mallett is molasses slow and it presents a glaring problem. Like Rob says in his post it's something that a team will have to live with and work around at the next level, but I do think it's manageable. And if he's able to develop any of Big Ben's unsackableness, something he has both the arm and size to learn, he could be unstoppable.