Quick note: BitTorrent 4 has been released. You don't need to do anything just yet, but you might want to start looking for updates to your BT client of choice.

The biggest change: BT traffic will now be marked with the "bulk data" tag. Supposedly this'll improve download speeds and lessen BT's impact on latency-sensitive applications like VoIP. In practice I'm guessing it'll mainly be used by ISPs to throttle your BT download speeds -- so there's another reason to wait. I'm sure somebody will offer a configuration option to flip the bulk data tag off.