I think this is great actually. I don't think they are selling out at all. If anything they are keeping their legacy alive. Embracing new media like this is not a bad thing. As Geddy mentions, the Guitar Hero / Rock Band franchise is a great launching pad for discovering new music.
I also think Rush is not getting any younger, and royalties, etc, from these type of things are a way to help fund their later years.
Although, I have been a "real" musician in the past (Bass, Keys, some drums, some guitar), these "Rhythm Games" are still fun to play. Some musicians thumb their nose at them, but fun is fun. I have the entire Moving Pictures DLC on Rock Band and drumming along to the songs pretending to be the great Neal Peart is a blast.
Finally, these rhythm games are progressing. Rock Band 3 for example has worked with Fender to use a REAL GUITAR. Now you can learn and REALLY play the songs.
It's all good IMHO.