What does Android 2.2.1 bring?
I'm not aware of an official changelog anywhere, but you can look at the commit log on the git repository here. Anything between the 2.2_r1.3 tag and the 2.2.1_r1 tag was a change added in 2.2.1.

The summary: it's basically just bugfixes, hence why it's a smaller increment (as opposed to 2.2-&gt;2.3, for example).

Note: If you're getting a 2.2.1 update from a carrier then they may have added other changes to their own software, so the above applies to the Android OS itself and not any additional components.