Of the Android devices that have access the Google Play store in the last 14 days, Google reports that 10% are running Froyo and another 4% are running a version older than Froyo. However, I imagine the number of Froyo devices is even higher if you include crappy devices that do not use Google's Play store.
More telling to me is a browse through the smartphone section in big Tokyo electronics store.
There are probably on the order of 30-40 different models, and I'd say at least half say "OS: Android 2.0"... :(
[Those are typically older models, of course, but with new phone models released constantly, the oldest are not more than about 1 - 1.5 years old. Also, some of the models I saw running 2.0 looked pretty up-to-date hardware-wise. It seems manufacturers are not always so keen to use the newest OS version...]
https://developer.android.com/about/dashboards/index.html