I'm working on a project (Java SE 5, Swing desktop app) that currently uses Hibernate for it's ORM solution. We're reworking some stuff (because we failed to keep good separation between the persistence layer and the UI layer). In the rework, we would like to move to a strictly 'portable' JPA-based solution. In other words, we would like to use as few Hibernate-specific features as possible. We still plan to keep using Hibernate for now, but in the future will probably move to a Java EE 5 architecture where we will be referencing EntityManagers and PersistenceUnits (even though we might still use Hibernate as the persistence provider).

Are there any nice websites that might give us a head start on this migration?

In case it matters, right now we're using HBM files for our mappings instead of annotations, so we'll need to convert our HBMs to the JPA schema.

I think you are on the website you are looking for. Keep digging around on this site. I continue to find more goodies here. Here are some particular examples that can help with your want to code JPA compliant as much as possible.

The sample chapter:
http://www.manning.com/bauer2/chapter2.pdf

The Hibernate Annotations and Hibernate EntityManager sections found on this page:
http://www.hibernate.org/5.html

I have found that the Hibernate documentation above does a nice job of explaining JPA and Hibernate specific implementation and extensions.

Also, the Hibernate wiki on this site has lots of 'outside' references:
http://www.hibernate.org/37.html

Hope this helps,
Jeff