They've introduced S-Gen which is a good, precise, generational collector. I don't know if it's the default, but it is considered production ready IIRC.
It'll be the default in Mono 3.0 (or 2.12, whatever we decide to call it). Currently, in Mono 2.10, Boehm GC is the default, although we've been using SGen for our commercial products (which is why we feel that SGen is now ready to be the default).
