Occasionally Google produces charts showing "the relative number of active devices running a given version of the Android platform." The latest of these charts is special though: It apparently marks the first time Android 2.1 was the most used version.
It's worth noting that this data was compiled "based on the number of Android devices that have accessed Android Market within a 14-day period." So don't run off shouting about the death of any Android versions older than 2.1 just yet. [Android Developers via Android Police]