Google Android developers site has recently published information about the current distribution of the Android versions providing data about the relative number of active devices running a given version of the Android platform.
As per the data, "87.4% of the Android phones use Android 2.x, up from about 55% in July. 51.8% of the phones use Froyo, a version released 7 months ago. In only one month (August 2010), Froyo's share grew from 5.5% to more than 28%, after Motorola and HTC updated phones like Droid and Desire."
The following pie chart and table is based on the number of Android devices that have accessed Android Market within a 14-day period ending on the data collection date noted below:
The following stacked line graph provides a history of the relative number of active Android devices running different versions of the Android platform. It also provides a valuable perspective of how many devices your application is compatible with, based on the platform version.
On the other hand, ZDNet says that "almost 13% of Android users are still running versions of the mobile OS that're several generations old and these users will never see Android 2.1", but this percentage is constantly decreasing. Even if they're using Android 1.6, that's still a much better operating system than the one from a feature phone and it still lets them use many apps from the Android Market. A slower update rate is a small price to pay for creating an ecosystem of heterogeneous devices that run the same operating system which isn't perfect, but it's "good enough".