One reader reports that a Yellowpages.com representative claimed that Yellowpages.com would soon be arriving on the iPhone as a new application similar to Youtube. It is reportedly due sometime next month, presumably though an iPhone software update.
Yellowpages.com is a telephone directory owned by AT&T. They already have mobile versions of the site available for a limited number of AT&T mobile devices. It certainly seems conceivable that an iPhone specific version could be in the works.