If you're wondering about what screen sizes and flavors of Android are currently out in the wild, the Android Developers site has a handy little chart of both (each image below is hyperlinked to the page):

So, as far as platforms go, there aren't many people still out there running less than 2.X... and you'd probably be safe not worrying about Small or Low Density screens.

I'd be curious to know if any of the other app stores (Amazon, Archos, etc.) offer similar stats...