This actually happened a couple weeks ago, but I can't see that anyone covered it but me.
According to a couple tracking sites, the Android Market now has more apps than iTunes. The margin is not very wide, but it's real. The Android Market now has just over 480 thousand apps listed, and iTunes has 460,000.
Curiously enough, in the 2 weeks since iTunes fell into second plac, the Android Market added 20 thousand apps (approximately). In comparison, iTunes added only 5,000 more apps in the last 2 weeks.
The story is actually more interesting now than it was 2 weeks ago. The Android Market is growing 4 times as fast as iTunes. It's not only unseated iTunes, there's also a pretty good chance that iTunes will never be able to catch up.
BTW, the Amazon Appstore had around 10 thousand apps in late May. If you were thinking that it was a serious competitor , think again.