There's been multiple APK support (for different OS versions, etc.) in the Android market since last July[1], so you could do the best thing for your "main" version in one build and have fallbacks for other versions in other builds (where necessary, of course).
[1] http://android-developers.blogspot.com/2011/07/multiple-apk-...
4.0 will see a large spike in the coming months.
Nonetheless, simply looking at the pie graph isn't enough -- if your app targets higher end phones, for instance, aim high: Cut out the derelicts and the abandoned devices, and let their vendors suffer.
Too many Android app providers try to target all, and in the end please none.
http://developer.android.com/resources/dashboard/platform-ve...