I have heard, but not confirmed, that if no block is solved for 15 minutes difficulty will drop. I'm not honestly sure if that's bitcoin or another alt-chain.
Thanks for the reply, coinerd. Interesting behavior, but it would be very difficult to take advantage of it, unless most of the miners (or software) are controlled by someone.
I just registered. This seems to be THE community to discuss about BTC. These are exciting times - it's going to be interesting to see how the currency develops, and if the banking powers will be able to control it.