The rules engines in 2006 is a lot like the one in 2004.
I think MS has a virtual lab on 2004 (http://msdn.microsoft.com/virtuallabs/biztalk/).
Oh, I just noticed they have a lab for rule in 2006 on the site – even better.
Here is a white paper covering rules in BizTalk 2004. That might help. Can probably find it via Google.
Other then that and the help docs, there isn’t much on the rules engine out there.
Best of luck.