This is great news. I'm not sure how much Rob will be playing guitar from now on, but you guys should have a 3 guitar attack for some of the more layered tracks. I don't know if it's sonically possible or viable in a live setting, I'm just throwing the idea out there. I think it'd be pretty awesome if Tomi learned some of Jonne's dual lead sections like in Cybergenesis, Nova Prospekt, Violator, and many others. They're both Jackson players and they're both greatly skilled, it could be done.
Then again, maybe this move was done so that Rob can focus 100% on vocals again.