Depends how much you care about reach and what type of app you are building. For most apps it's probably advisable to use at least Android 2.3, or even 2.2 if y

Depends how much you care about reach and what type of app you are building. For most apps it's probably advisable to use at least Android 2.3, or even 2.2 if your app doesn't depend too much on API's from 2.3, which I don't think added that many new and important API's, except for games. If you want to build a bleeding edge, highly polished app or game that can take advantage of the latest Android API's, and turn that into important features of your app, it's probably alright to go just with Android 4.0. If you do it like this, make sure to take advantage of the "fragments" in Android 4.0, to make the app look good on tablets, too.