Linq for NHibernate is not part of the 2.0 release. Linq support is planned for the 2.1 release. That said, we have been getting a lot of questions about that.
The technical reasons are not really interesting, but suffice to say that to provide good Linq support we also need to modify NHibernate slightly. Those changes happens on the trunk, which is what Linq for NHibernate is following.
Now, the disclaimer. We are not supporting this. This is an unofficial (but welcome) contribution from the community. It is likely to have bugs (in fact, we know it contains bug in the more complex Linq queries, that is why it is still under active development), and the NHibernate team response is most likely going to be, use the latest, which requires the NHibernate trunk, not the NHibernate 2.0 version.
Use at your own risk, etc.