If Roddick's ever going to beat Federer, it's going to happen tomorrow (when it counts for nothing). But I expect that Roddick won't trouble Federer if they meet in the semis of the AO.
I think it's in Federer's best interest to beat Roddick tomorrow. A defeat so close to the AO, with Roddick knowing that he could meet Fed in the semifinals, would be slightly deflating, even if it's only an exhibition.
It's an unofficial match and doesn't count towards the head-to-head, but it could help Federer maintain his mental stranglehold on Roddick.