Some of his problems with touchscreens include: sun glare (MacBook users are already intimate with this one), grease, lack of one-handed operation and the potential for trapped dirt.
As an avowed QWERTY guy I seriously dig the hardware keyboard on my palmOne Treo 700p. Granted, it took some getting used to, but I can practically touch type on my Treo these days. My wife uses a Sidekick 3 and I swear that she can type on it as fast as a regular keyboard.
I have to admit to having some serious trepidations about the iPhone's upcoming all-touchscreen interface and Powell's piece isn't helping!