A recent survey, by Becta, showed that around 40% of colleges are using Moodle for their virtual learning environment (VLE).
I don't know of published figures for Higher Education, but believe that the use is probably about the same. If you are one of those who use it, are you aware that there are a bunch of webparts available that integrate it into SharePoint? What's the point of that, you may ask? Well, for the same reason that lots of other VLE providers are building web parts for SharePoint. It means that you can choose a single portal architecture (ie SharePoint) and integrate your different systems into it - email, student management systems, VLE etc. And if at some point you want to change one of those systems, you can do that by changing your webparts, not your whole portal.
You can find the Moodle webparts on Codeplex, a 'community source' initiative.