My understanding is that the difference between 1.5 and 1.6 was huge in terms of what it provides in API/functionality and a lot of apps target 1.6 as a minimum

My understanding is that the difference between 1.5 and 1.6 was huge in terms of what it provides in API/functionality and a lot of apps target 1.6 as a minimum as a result. Probably a lot more than target 2.1 or 2.2. 2.0 almost doesn't exist, it was upgraded so quickly on the devices that had it. My experience is that if you're on 2.1 you can run most of what's on the market. I haven't used 1.5 or 1.6 enough to know about them (other than technically), but I wouldn't be surprised if 1.5 can't run a large chunk of the market and 1.6 probably gets excluded from a lot of cool stuff, too.