WoW leading the charge? Check into Eve Online and you'll find that they've had it for a while and LotRO was designed with the function built into it's UI.
Good call on EVE Online, I missed playing EVE so I didn't even think of it. I knew about LotRO integrated chat function, however I am globbing the VoIP bandwagon around WoW as it appears every MMOG is slapping it into their builds (Tabula Rasa, Warhammer Online, et al.) at the same (relative to release) time. I guess the main question is, "is this good for MMOG's and more importantly is it good for the players?"