NHibernate and "The method or operation is not implemented."

I've had a minor set back.
Group joins don't seem to be implemented in version 3 of NHibernate. I found a blog that stated they were going to be added, but that was in November last year.
By my reckoning I have 2 obvious options.

1. Swap out NHibernate for Linq to SQL but put in a common interface so I can swap it back later
2. Re-work the linq code I have already developed and am re-using

I favour option 1 right now, which I thought would be a "Nice to do" right from the start.