The Unofficial Apple Weblog (TUAW) has a short article on adding touch to an 20″ or 24″ iMac.
http://www.tuaw.com/2008/06/18/troll-touch-your-imac-screen/
Touchscreens are often hailed as the downfall of keyboards and mice. Their usefulness in situations such as kiosks is well established, but the practical design concepts needed to transition the technology to a complete desktop solution are limited at best.
Adding touch into an environment creates many new interaction concepts that do not exist with a keyboard/mouse. Many concepts from the keyboard/mouse experience do not translate well into a touch environment either, making combining the two technologies difficult.
The technology is very interesting, especially as a step to bring touch “to the masses”. At $900, my iMac will remain touchless for now though.