The announcement is on the users list - http://groups.google.com/group/mongodb-user/browse_thread/thread/62b071549a95dd4a?hl=en
Until now the two offerings were Norm and mongo-csharp, both are excellent OSS projects with lots of contributions and very nice velocity. My concern there was always that although there is definitely a place for more than one flavour, as usage patterns and even personal taste differ and pleasing everyone in a single product is impossible (see on rubyland for e.g., – there are MongoMapper, and Mongoid, and there are even some more, less-widespread ones). The major difference is that since the core of the ruby driver is maintained in a single location (and backed by paid-developers thx to 10gen), the things that are the same across (mainly BSON, client-server setup, connection management) are not duplicated, so we get a fully featured, very robust, tested by many core, and it gets out very fast.
On the c# side of things, the implementation of replica-sets in the client took some time to emerge after the official support on the server side was out.
So, it is an exciting announcement for the .NET community. I hope that the current drivers will know to adapt the drivers to use the official core.