On Wednesday of this week, Yahoo changed its instant messaging service in such a manner as to prevent third-party services, such as Trillian from interfacing with its services. The move is, of course, claimed to be against spamming, but it may be enough for Yahoo to reclaim their system and boot invaders like Trillian out for good.
Yahoo (and Microsoft) blocking Trillian out is not new, however...
"This time Yahoo said it will continue changing its protocols to prevent clients such as Trillian from finding new ways to incorporate Yahoo. Again, the measure was cited by Yahoo as a way to prevent IM spam."