With my portable 17" Multi-touch screen I recently built, I have been using the 2.1mm FishEye lens on my PS3 Eye camera with great success.
I have just started a 32" DSI setup and the 2.1mm lens is too distorted to detect blobs on the edge of the screen.
So I recently bought a pack of 6 lenses from DealExtreme to try different angles.
The product is called "2.8mm~16mm Fixed IRIS Lens Set for Webcams and Security/CCTV Cameras (6-Lens Pack)" and it costs USD$14.91.
I have posted a screenshot below of the output from the PS3 Eye using the different lenses.