No problems: I've dropped a couple of slots on my primary keyword, but I've got a bit of a 302 problem from a couple of directories and an authority site has jumped ahead of me with two pages on my subject (I'm still in position 3 and 4). Other keywords are fine if not better, and traffic from Google and MSN has increased. The site has always done badly in Yahoo, no change there.
Of course, the only search engines I know support this tag are Google, MSN and Yahoo - it won't block caching by other sites.
Cloaking ;) Either add it for known bots, or do what I'm doing for a new forum I'm launching soon and make it appear only when a user is not logged in.