If you are using the kit frame, AndyMark sells a bracket that fits over the frame rails:
http://store.andymark.biz/am-0317.html. Last year we had one of our sponsors bend up some channel for us that fit over the frame, and I found out about these later... although getting them donated by a sponsor is cheaper
We secured them with 1/4" hitch pins through the vertical holes. Carrying the robot by the bumpers was no problem. I was able to remove a bumper in less than 30 seconds when smoke started coming out of the robot while doing some testing.
If the GDC goes back to requiring "bolted fasteners" (or whatever the wording was in 2008), then a 1/4" bolt can replace the hitch pin, but that takes a little longer. As a robot inspector, I really (really, really) hope that they do not go back to the 2008 requirement. Being able to remove and install the bumpers quickly has a significant impact on both how smoothly your inspection goes, and also on the ability to perform maintenance and repairs on your robot.
Ok, let me say that again: whatever method your team uses to attach the bumpers to the robot, you should be able to remove and install them quickly. In fact, the rules last year recommended that you should be able to complete the process in less than 10 minutes. I have seen teams that it takes an hour, and they are still not what I would call "solidly attached".
There are several excellent ideas presented so far in this thread, and I hope to see even more. The biggest thing that I see in the good ideas vs the bad ideas that I see every year, is that the good ideas take bumper attachment into account during the design process, and the bad ones treat it as an afterthought.