Just posting a link here to another Live Journal Community for writers.  Thanks to Kat for the heads up, go here for a very helpful post on both finding and being a beta.  http://community.livejournal.com/shared_wisdom/10950.html

One of the things that struck me was her suggestion that betas stick to writers in their own personal comfort zone. Not that you couldn't or shouldn't do something short that is outside of your usual area, but that it is often difficult to do a really good job for someone who is writing in another fandom or area than what you are interested in.  That question has come up on this site and I think it's useful to have validation that it is all right to say, "Thanks for asking, but I don't think I'd be comfortable (or happy) doing this story."