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Have a look at the third post from Chicago in this thread
That will give you more information then I ever could about what advertisers may be looking for.
You might also look into a few of the cutting edge features that have been rolling out with some IYP's such as Pay-Per-Call or SMS info.
Are you just listing the businesses like a normal phone book, or are you tapping into other revenue streams such as shopping? An example would be ShopLocal or how SuperPages does it.