this topic is certainly nothing new to the boards as speculations for an iMac touch are increasing.
besides the obvious technical difficulties, desktops just aren't good for continued, multi-hour touch use. try holding your arm up for around 5 minutes and do various finger exercises, i bet you wont last long.
personally, i dont really care for a touch screen iMac - i'd much rather have a redesigned Mighty Mouse.. ya konw.. something feasible.