<p>Is there any supported way of mapping datetimeoffset the new datatype in SQL 2008 to the System.DateTimeOffset using nhibernate?</p>

(Assuming google didn't turn of anything)
You will probably need to create a simple IUserType to handle this, also I am sure that the NH team would gladly accept it as a patch for everyone to benifit.
Ayende has a good post
here
on this.  He goes though a couple of options to solve his problem so you might want to start from the bottom of the post (not the comments) and go up.