3 theories that make a lot of sense to me:
1) Shark-skin suits
2) Depth of the pool (the pool in China is 3m as opposed to 2m) - more water to displace means less turbulence in the water. (fact check needed!)
3) Maximum velocity for a swimmer occurs immediately after a kickoff following a turn (obviously). However, now you are allowed to do that flutter kick (and maintain that max speed) under water for as long as you want. Before the games in Athens you were allowed only to do 2 flutter kicks and then you had to break the surface of the water and start the stroke.