No dice, my friend. Looks like something Apple should think about for iCal 2. Not to mention sharing calenders over rendezvous.
It should also be noted that the colored iCal event boxes are currently drawn from composite images that are tiled/stretched. That's why there's not much room for customization. I hope they move away from that sort of implementation, and go straight to drawing the event boxes on the fly, with vector objects. After all, they're not very complex shapes. And that would open the door to choosing custom calendar colors and such.
What i'm doing in the mean-time is specifying colors by category instead of trying to distinguish every calender from one another.