GOOD QUESTION!!!!
I've been considering that for my Miata, which has been recieving a daily dose of sandblasting because of all the highway construction underway around here. The plastic front bumper is a now a pitted mess from all the stones, sand and everything else. I haven't even considered doing anything to it until all the road work is done, but it shouldn't be too much longer.

Having a little bit of experience with this paint so far, I'm optomistic that it will adhere to plastic bumpers well, but have no concrete evidence yet. I just bought a new car for my daughter tonight and will have her old POS Ford Escort toclean up for resale. I had to replace the front bumper and fender on it last year (courtesy of my son!) and just did a quickie rattle can spray job on it. It's white, and I just happen to have some white Rustoleum left over from Project Beer Fridge. If I try it out (even on a small section) I 'll make sure to post some results. The Escort bumper, although plastic, is significantly stiffer than the Miata bumper. Could be interesting. Anyone try this?