Title says it mostly…
While it would be nice to be able to cover 90%+ of devices in use now, if it starts to complicate things, I think, focusing on devices being released now and soon clearly has priority.
Bonus points, on guidelines to set project settings and emulator to be in sync and without warnings.
See the Android Dashboard for stats on API usage and trends: http://developer.android.com/resources/dashboard/platform-versions.html
(As of May 2011, it looks like targeting 2.1 and 2.2 covers about 90%, but obviously this will change.)