Touchscreen Photoshop would be a giant pain in the ass. A mouse and even a good tablet can give me pixel-level precision at 1x zoom, trying to do that with a touchscreen when my fingertip covers half a hundred pixels even on a large monitor? No thanks.
I've seen some interesting things done with multi-touch screens, but I still don't buy the propaganda that they're a million times more convenient than a mouse for general tasks.