The "QR app" iPhone application, on an iPhone 3G, can read a 200x200px QR code encoding "http://www.hubmed.org/" when printed (32x32mm), but not 100x100px.
A longer URL encodes more information, so a 300x300px QR code is better, though it's possible to use a 200x200px code if you're careful with the camera. Using a URL shortener would solve that problem, of course.
Apparently the iPhone 3GS can read 15x15mm images, i.e. 100x100px.
QuickMark ($2) seems to be the best QR code reader for the iPhone - it automatically detects codes in the current camera view without having to manually capture an image.