Disturbingly 10gen uses SSL support as a reason to use their subscriber packages. Sure you can build it in yourself if you compile the package, but it's disappointing that SSL support is one of the carrots that they use for their premium offerings.
SSL support is in the free distribution but you must built it yourself. One reason would be export controls; another is that creates a dependency on the SSL library for those who don't use SSL, which we found awkward (especially if doing all platforms; the subscriber build just does the popular ones).
So it's available; albeit there is an intent to have a subscriber build with some extra features that are heavily enterprise biased in their usefulness.
Edit: Link to applicable docs on how to compile in/use. http://docs.mongodb.org/manual/administration/ssl/