I keep hearing that, but to make that argument, you assume that it makes sense for Apple to build in a very expensive screen and write some very sophisticated software for it (and presumably extend this functionality to the rest of their computers) just so you could occasionally move a window out of the way by fingering it rather than using the mouse.
The cost of multi-touch is such that if they ever implement it in a computer, it'll be a persistent, overwhelmingly useful (and cool!) technology - otherwise, why bother?
iPhone has a touch screen not because it's occasionally more convenient to use it, but because it's the ONLY way to use an iPhone. This solves the problem of having a (relatively) large screen in a (relatively) small device, so it's worth the extra cost. Putting multi-touch in a vertical iMac wouldn't solve any problem while raising the price significantly.