src/mongo/util/concurrency/mutex.h describes several mutex classes that are similar to boost::mutex, or implemented in terms of it.

There is no clear advantage to these classes in the C++ driver codebase, and we could possibly standardize on boost::mutex here. Risk is that it creates a lot of point divergences from the server code, upside is that we drop one more set of platform dependent code and push that out to boost (or C++11 when we pick that up).