True, RedOctane also has a modest game rental business and Harmonix is also the developer behind the successful Karaoke Revolution series, but there's no question that Guitar Hero was the game that sent these two companies fortunes into the stratosphere.
The MTV deal, as reported by the New York Times, will utilize Harmonix's experience with music-based games to design play-and-remix-and-sing-along experiences for MTV's web, mobile and VMTV virtual world services. That's all well and good, but we'd like to see what this partnership can do with the console-based MTV Music Generator series, which has lagged a bit since CodeMasters took the publishing rights over from... Activision. Hmm...
[Update 1: Harmonix has issued a press release with more details about the buyout]