Over at Quirky they are working on a collapsable helmet project, its in the "Concept Phase" at the moment. There are some nice ideas - I pinched some and have shown them below. I hope this project goes all the way, for sure carrying around a helmet before and after cycling can be a bit of a pain, so if your helmet could decrease in volume for easy stowage would be great. Dahon has a folding helmet, the Pango which is pretty well thought out. Lets see if Quirky can do better.