If your supporting 2.1 you may as well support 2.2 and double your customer base. I don't see why you should leave out 1.5> though, unless you find something you specifically need in the API's.http://developer.android.com/resources/ ... sions.html
2.2 gives you the best performance updates, but this is the OS not your code so no need not to support 1.5.
I haven't read of any security issues as of 1.5>
They're are no limitations of 1.5 that are MAJOR
drawbacks, you can do most things in this version. Although you can do some of them easier
But you can also just limit the functionality of 1.5 uses whilst still giving them an app to download (and also a reason to upgrade!) using reflection.
Check out this presentation:
Casting a wide net for all Android devices:http://www.youtube.com/watch?v=zNmohaZYvPw
This gives you idea's about using reflection to allow your 2.2 users to have the latest features whilst still supporting other versions.
On a side note, if you want to view more of these presentations, especially on your phone! Check out:Google Dev Helper