Not that I have seen. What I saw was that apps had to be tested for 3.0. In fact, an app I summitted a week ago for 2.2 was approved this morning.
The iPhone 2.2.1 sdk is still available on the front page of the dev center.
I have several SDK's installed. The 2.21 sdk will build for 2.0, 2.1, 2.2,and 2.21.
The 3.0b5 would only bulid for 3.0b5.
The 3.0GM builds for 2.21 and 3.0GM.
Since not everyone will want to upgrade to 3.0, it would not make sense to target that OS unless there was a specific feature you wanted to take advantage of.