According to the Platform Version statistics at the Android developer site, 37% of devices are running 1.5, 29% are on 1.6, with 32% running 2.1 - the latest version. Android 2.1 has been available since January 2010, however the number of users on older phones is shockingly worrying and is harmful for the future of Android.
I'm not too bothered about the amount of different devices which use the Android operating system, which certain people seem to focus on. What does concern me are the slow handset manufacturers who aren't updating their phones to the latest version within a reasonable amount of time. The Motorola Droid was one of the more successful Android 2.0 devices which has been updated to 2.1, which probably explains why there are still a few users still on the 2.0 series (inexperienced users who don't know how to update), but could be much worse if this device wasn't updated.
Manufacturers need to update all their recent phones within a reasonable amount of time, otherwise the future of the platform is going to be an awkward one. Users are often spending hundreds of pounds on phones which they consider to be smart phones, yet their manufacturer is treating them like the existing dumb phones - where you get updates if you're lucky. Customers are paying a premium for what they think is an alternative smart phone to the likes of the Apple iPhone. Smart phones deserve a respectable lifespan of updates after the release.
At the moment my HTC Hero is due an update in June 2010, this is 5 months after the release of 2.1 on other platforms. I love Android, however I no longer like HTC. I'm sure there are others who love the operating system, but hate the way their manufacturer is treating them with updates.
Just remember HTC - other phone manufacturers are releasing Android powered devices too.