After reading a bit on car keyfobs I have a project idea I might have a go at.
I read
this old article on HaD, the first comment on it is what if they just jammed the signal and the owner doesn't check to make sure the car is actually locked. Broadcasting strong noise on the 300-400 mhz frequencies could drown out the signal.
LadyAda has designs for something she calls
Wave Bubble. The BOM doesn't give prices on the page and I'm too lazy to figure it up myself, I also don't have an oven reflow for all the SMD parts, but still.
Wave Bubble would be cool, but a device just for cars would be cheaper and simpler.