There was a post by Brad on Elemental Support- when he said support for a sequel was pushed back to work on Elemental.
To me, that pretty much confirms GC3 will be next (and probably was going to be for SD's main team- which is why I think the 2nd studio was propsed- they had two good ideas and only manpower for one). Also, with Elemental's launch problems, it would be the safe play to go to your most reliable IP afterwards.
GCIII, working on the Elemental engine which will have been refined, and with some of the gameplay idea from Elemental, sounds really awesome. I'd say I can't wait, but I"m sure Elemental if SD improves it constantly will make me very patient.
BTW given Brad's comments- I wouldn't expect a GCIII release officially until late 2012 at the very earliest. More likely 2013.
Being a fan of SD stuff sometimes requires a good dose of patience. 