<p>What kinds of considerations are there for migrating an application from <strong>NHibernate</strong> 1.2 to 2.0? What are breaking changes vs. recommended changes? </p>

<p>Are there mapping issues?</p>

Breaking changes in NHibernate 2.0
If you have good test coverage it's busywork.
Edit: We upgraded this morning.  There is nothing major.  You have to Flush() the session after you delete.  The Expression namespace got renamed to Criterion.  All these are covered in the link above.  Mappings need no change.  It's quite transparent.  Oh, and transactions
everywhere
, but you were probably doing that already.
By the way, here's an interesting look at the changes:
http://codebetter.com/blogs/patricksmacchia/archive/2008/08/26/nhibernate-2-0-changes-overview.aspx