The 100 fly looks to be Phelps' toughest individual race. The guy from Serbia has looked great in the heats and semis (swam the 2nd fastest time ever in the heats), and of course Crocker has the world record. Phelps will be going up against 2 guys who have swam this race faster than he ever has, and both of them have been sitting around waiting for this event while he has been at the pool pretty much all day every day since the Games started.
The 4 x 100 medley isn't a slam dunk either; the Aussies have faster breaststroke and freestyle legs than the Americans, at least on paper.