Quoted text in e-mailed replies is a big problem. But the most insidious problem of all is "out of office" messages. They're sent by the recipient with all the proper hashes, etc, and there are so many variations on the text that I don't think it would ever be possible to strip them all out.
We had one hosting client who had this happen to them. It turned into a huge feedback loop (each notification containing the out of office reply is replied to again) and completely backed up incoming mail, not to mention really junking up the forum.
Maybe it's solvable, but I never figured out how to do it. I think the best you can hope for is getting the really obvious ones, like "out of office reply" and "vacation".
