We collect anonymous statistics and Ubuntu account for the majority of users who opt in. The stats aren't publicly available yet for reasons unknown to me (likely lack of manpower).
The last figures I have put Ubuntu at 73% of all users. Admittedly these are from right after we enabled the feature in the 1.5.4 release and Ubuntu is simply an easier platform for users to experience development releases of Banshee which could bias the number. However since I read every incoming bug report and the same data regarding the underlying system is present in the logs this number does not seem far off.
The vast majority of our users are on Ubuntu using any way of counting them I have available.