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 schools, but believe that the use is significantly lower - single digit percentages. 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 a school can choose a single portal architecture (ie SharePoint) and integrate their different systems into it - email, management information system (MIS), VLE etc. And if at some point they want to change one of those systems, they can do that by changing their webparts, not their whole portal.
You can find the Moodle webparts on Codeplex, a 'community source' initiative.