Do you use the online Yellow Pages? I do. My favorite yellow page site is Dex Online (used to be known as Qwest Dex, but I think this was one of the assets Qwest had to sell off). In any event, I read an interesting post by Seth Godin in his Blog regarding the Yellow Pages online. Seems Seth was searching for a magic store near his house and the couple of sites that he searched and couldn't find the magic store that he knew was there, which shows a weakness in the whole online yellow page system.
I would think that it wouldn't be too hard for Amazon to use its search inside the book technology for the yellow pages. Think about it, Amazon ships all the yellow pages over to India, have the spines ripped off, and scan in all the pages. Then Amazon programs some very basic code to recognize the categories (they could use the pre-existing yellow pages category classification), and suddenly you have the perfect blend between the printed and online yellow pages. Classification and drill-down to the level of the printed version, but instant online access. Currently Amazon doesn't allow you to page through an entire book that they have scanned in because of copyright concerns (i.e., a person being able to pring out an entire scanned book), but they do let you scan through several pages past your search term. Similarly, Amazon could let you page through the number of pages that contain the category you are searching under.
By the way, this isn't something that only Amazon could do. One has to imagine that the yellow pages exist in some sort of digital fashion before they are typeset . . . Let's see, if you were a yellow pages company you could license the Amazon search technology (like Apple licensed the Amazon One-Click technology) and only produce your yellow pages digitally (or at least print less copies or give the consumer the choice of how to view the yellow pages). Imagine the possibilities . . . dynamic ads rather than static ads in a printed book, real-time updating (if it hasn't been scanned, then it's still digital, which means it can be updated) . . .