Last night at the OSS panel, it was asked what are the indicators for a healthy OSS project. In my opinion, it is the size of the community and the number of active committers.
A good example of a project that wasn't healthy is NDoc, when the lead (single?) developer left, the project was mostly stranded, and the community basically died around it. A good example of a healthy OSS project is Castle, where we currently have ~20 active committers, and a very active community.
By the way, by this metric, Rhino Mocks is not a healthy project, which also came up yesterday, if I happen to meet a Bus somehow, I am not sure that the project will continue, even though there are a large number of people using it.