It's because everyone in the world is downloading Angry Birds Rio at the same time lol....
Seriously though, I'm having the same problem. I, on the other hand, called Apple Care and wasted half an hour with them trying to diagnose my "network problems" despite telling them over and over that my MacBook Pro, iPad, and iPhone were all having the same problem, BUT my girlfriend's account worked on the MacBook Pro and her iPhone connected to the same network. I love AppleCare , but their phone support is always so reluctant to admit it could be a problem on their end.