I subscribe to Make Magazine, a magazine dedicated to hackers, and saw this in an article. Sugru is a air curable rubber that is pliable like clay and dries like rubber. I'm thinking about ordering some to add rubber to my foregrip and to the plastic grips on my newly acquired hyperframe. I'll post a follow up after it arrives and I've had some time to play with it.

http://www.makershed.com/Sugru_8_Pack_p/mksu1.htm