Looks like in another 2-3 months, the market share for 2.3 support will finally reach 75%. If I was starting today, I would be comfortable targeting that API level. Wish I could say 3+ was doing so well...
http://developer.android.com/resources/dashboard/platform-ve...
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.
It still runs in the Dalvik VM but you don't have to write a wrapper Java Activity - everything is C or C++.
For certain things, particularly opengl games, this can make a lot of sense.
[1] http://developer.android.com/reference/android/app/NativeAct...