My DB schema has a string as a varchar(max). I have read the other questions concerning setting Length to more than 4000 or 8000 so that it really generates a (n)varchar(max) in
It would appear that this is an old problem which is now resurfacing in NHibernate 3.x builds; you can read about the workarounds here.
Note: I have updated the original link I posted as it was outdated.