I take it back. Thanks to this post by Ben Nadel, I found a way to make the number keyboard come up for numeric fields. I can't guarantee that it'll work in all versions of the iPhone, but it worked well on my 3G running OS 3.1.2.
@pb great thanks! This is a really nice improvement for the Fuelly Mobile site. It was really a pain when it always brought up the keyboard instead of the numeric fields by default. Go Fuelly!