I'm planning on building an add-in using ArcPy, but some of the functionality I need is only available in 10.1. I want to distribute this add-in and was wondering if making it 10.1 only would limit adoption. So would like to know what percentage of ArcGIS users are using version 10.1 already?
No one else seems prepared to take a stab at this one so I'll "upgrade" my Comment to be an Answer.
It's really hard to say but I think it would be in the range of 50-70% for Desktop (Server would be lower than Desktop because its installation is recommended first).
I target 10.1 for anything I am doing, and if it also applies to 10.0 (which it almost always does) then I consider it a bonus.
One way to derive some semi useful stats may be to use the Stack Exchange API (like Bill Dollins did for a different purpose) to graph the change in ratio of arcgis-10.1 to arcgis-10.0 tags over time. More questions should roughly indicate more use.