As others have already realized, touch is not the best method for many tasks people use computers for. That's why PDAs with touchscreens usually also have navigation keys. Multiple tools for multiple situations.
Can you imagine writing a term paper on a touch keyboard, and trying cut/paste?
Can you imagine trying to use a drawing or CAD program?
Can you imagine if spreadsheets had to use REALLY BIG CELLS just so you could touch each one more easily?
Even Tetris is easier if you can leave your fingers lightly on top of real keys for physical guidance. Something you can't do on most touchscreens.
I'm typing on a touchscreen laptop right now. Do I use the touchscreen? For debugging my apps, yes. Very, very seldom do I actually reach up and tap a web link or something. (When I do, the look from some people's faces is priceless, though
A mouse is still the best input tool for most jobs. Tablets/pens for others. Touch for some. Biggest trouble with touch are: it's inaccurate, and your pointing device covers the target. It's always best to let the user have choices.
The iPhone, in its current form, has an extra handicap for enterprise appplications since it uses a capacitive screen... no optional stylus input for signatures, drawing, and so forth.