The more I think about it, the more the idea of Miller as Spartacus makes real sense to me. He's attractive so he'll work as eye candy. He has the requisite intelligence. Being of mixed heritage, he'll certainly be more Mediterranean than the Welsh-born Australian Andy Whitfield.
The one question I have is whether or not he can physically portray this role. Whitfield was good in the role almost entirely because he fully committed to the silliness of the physicality. But it's unclear if Miller can be a physical actor in the way that Whitfield was. His innate intelligence – which worked perfectly for Michael Scofield – may undercut the tone that Starz has established for the show. It'd be interesting to see if he can adequately modulate that aspect of his persona in order to play the Spartacus of this particular show.
That said, it's irritating to me that he hasn't worked much since the end of Prison Break. He's just too fine and talented to go to waste. It'd be nice to see him on TV again. And if he pulls off a role that is very different from Michael Scofield, he should be able to write his own ticket after that.