Gonna second the Simple Green and recommend Super Clean, too. A little spray, some scrubbing, and a rinse, and you have squeaky clean rubber.

I used some tire dressing on my truck (don't recall what it was, and I gave it away), and it turned my letters brown. I guess I'm not really a big fan of paying for a look that makes me have to wash my tires more often and reapply the thing that made them look bad over time anyway... Ahh, jumbled thoughts.