With the RC33 update of Android slowly rolling out to G1 users across the country it's only fitting that Google has announced that the Android 1.1 SDK is now available to developers. Applications written with previous SDK versions will work fine in 1.1 but if you want to take advantage of the new API's in 1.1, this is the SDK for you.
According to Dan Morrill of Android Developers Blog, the update is "quite minor, but useful". Along with the new APIs, the 1.1 SDK has a new emulator image to let you test your applications. If you're interested in this kind of thing, the release notes are here.