There may be some big surprises in the pool. Michael Phelps barely qualified for the final in one of his strongest events, the 400 IM. He took the 8th and last spot, just out-touching Laszlo Cseh, who won the silver behind Phelps in this event in Beijing (and who was arguably the second best male swimmer in those Olympics). Of course, the best swimmers are usually not going quite all out in the preliminaries, just doing enough to make the final, but it should not have been this close. In Beijing, Phelps qualified first, IIRC, for most of his finals, and was certainly in the top three. His time today was way off his typical preliminary time in this event, and very far off his WR. He will now get an outside lane, meaning it will be harder to see the fastest qualifiers during the race.
Ryan Lochte didn't swim a great race, either, but qualified third. This was supposed to be a battle between the two of them, and might still be, but I wouldn't bet on it now. Overlooked in all the excitement over their head to head duals in the trials was that their times were well off (Phelps') world records. Both are in their late 20s when swimmers are traditionally past their prime.
I think Cseh must be in a state of shock. Weird things can happen in events which require qualifying heats. I remember in 1984 one of the favorites in I think the 400 freestyle barely missed making the final when he tried to save himself a little. In those days they had a non-medal consolation final for the top eight non qualifiers, and he not only won that event, but set a new WR, swimming faster than the winner of the gold medal event. I'm sure he would have traded the WR for the gold.