There has been a lot of discussion about this and the general consensus seems to be that while it sounds good in theory, in actual practice it would turn out to be tiring and counter productive.

For the novelty factor, good but from a practical stand point I think that all you would end up with is a greasy screen and a very tired arm.
Personally I do not sit within arms length of my iMac screen and to move the cursor from top right to bottom left, right now, I only have to move the mouse 10" but with the touch screen I would have to drag the whole 27" while reaching forward to touch the screen the whole time.

As this subject has already been discussed in these forums, I'll lock this thread and leave you with a link.
http://forums.cnet.com/7723-21564_102-565268/touchscreen-imac-will-it-be-true/?tag=mncol;4f

P