My old team did silk screen printing for the bumper numbers, which was donated by the same supplier that helped us with signage for the robot (as I recall).
This year the team I am working with does not have such connections established, so we plan to try using these white paint markers:
http://www.officedepot.com/a/product...t-PX-20-White/
I use them all the time at work, they mark on just about any surface, they dry in no time at all, and they're cost effective. Granted I haven't done it yet so it isn't tried and true, but I'd be surprised if the results were unsatisfactory. Will probably use some sort of stencil or projected image to get the numbers looking neat.