The Rose Bowl, if they lose their traditional matchup between Big 10 and Pac-10, get the first two available BCS teams of their choice. Even if USC loses to UCLA, which seems unlikely, they still get Michigan because they lost OSU to the BCS championship bowl. Then it's Michigan/USC.
If USC wins this weekend, it's hard to imagine a scenario where Florida could be a more attractive team to go against Michigan than LSU.
you're right to a point.
The key, however, is that if USC loses, chances are LSU is still going to move up into at least the #4 spot to clinch an automatic BCS bowl berth.
It wouldn't be the Rose, but it would probably be the Orange or Fiesta....