Trillian hooks bakc up to Yahoo Messenger. What a surprise.
Note to Yahoo: isolating mechanisms don't mean locking users out of deriving benefits (in this case, rare direct network externalities). They mean protecting resources from imitation or substitution, and hence, value appropriation by competitors. Add something that users value - which Trillian can't. That shouldn't be too difficult - considering you're Yahoo, and Trillian is, well, Trillian.