I was using a piece of foam rubber under my Arduino for the Marble Roller project. Not so nice, right?
Having a brand new MakerBot Replicator 2 on my desk, I decided to, well, make a nice bumper (or 3).
No design time needed as someone else did the hard work: http://www.thingiverse.com/thing:26237
Printed out wonderfully. Fits perfectly.