It looks like Yahoo have made good on their threats to lock out third-party clients; Gaim 0.68 is failing to log into Yahoo Instant Messenger. It may or may not be patched in the next release, and may or may not go up and down as the game of cat and mouse between the penguinistas and Yahoo's protocol cops plays itself out. On the positive side, it can still log into MSN, at least for the next week or two; after that, who knows.
If you're wondering where I've disappeared off to, that may be the answer. (Yes, I know there's an official Yahoo client. No, I'm not going to download it. Other than the question of installing ad-blasting spyware which serves a different master, my monitor's just not big enough to have one of each official messaging client, each with the usual k3wl look-at-me graphic design. And that goes for the 21" monitor at work too.)
http://gaim.sourceforge.net confirms that Yahoo have changed the authentication method to one Gaim doesn't speak.
I imagine the Trillian people have signed NDAs and done deals to get the documentation on the protocol. You don't suppose there's any chance of them porting to Linux (or indeed MacOS X)?