The buzz I'm hearing is that Portman actually wants to go to the Senate. Of course that plan hinges on Voinovich retiring which is looking less likely at the moment given his prominent position as one of the leading voices for the US Automaker bailout.
Indeed I've heard such buzz as well and Voinovich doesn't seem to be slowing down.
I see Portman as the GOP nominee simply because Strickland has a good chance of not appearing THAT vulnerable. Portman is the only one who wouldn't have go give up an elected spot to challenge Strickland and a failed run for governor would actually help Portman's future career instead of hurt it.