Hi all,
Yesterday I added this post in my technical blog: http://blog.minibofh.org/?p=203
It works like a charm. Search for VBTorOpenBSD relay in http://torstatus.blutmagie.de/ and you can see how fine it works.
I´ve tried to be so clear and explanatory (I´ve added even some screenshots) but anyway, feel free to feedback.
I hope it´s a useful doc for the community, because is a very cheap/easy/fast method to create a Tor relay just using a regular internet connection and a popular an easy virtualization solution as VirtualBox is.
:D