A company called NEO, mostly known for its DS and GBA flash carts and gadgetry in the homebrew/piracy scene, has made a motion sensor for PSP.
The tiny gadget (pictured) slots into the earphone jack of the PSP and enable the PSP, and draws power from the console itself. It allows the handheld to detect movement, likely using similar accelerometer tech to the Wii Remote.
It looks like a great little gadget but there's a catch: it doesn't work with any retail games. The device has been created as a tool for homebrew programmers to utilise in their bedroom programming creations. So it's great if you're running custom firmware and are into to homebrew scene, useless if you're not. It also won't work with the new slim PSP.
We can only hope that a developer takes notice and ceases the opportunity to produce a gadget like this for a more mainstream game.