Bonjour is handled by the NSNetServices and CFNetServices APIs, which you can read more about here. These are Cocoa and Core Foundation interfaces, and are publicly available for use in Mac and iPhone applications. As has been pointed out, many applications make use of this to do iPhone - iPhone networking (Game Kit is layered on top of Bonjour, for example) or Mac - iPhone data syncing (such as for Marcus Zarra's ZSync library).
Bill Dudney has created a stripped-down Bonjour server class for the iPhone, which he describes here. I created a matching Mac application which communicates with his test application and can be downloaded from here. Apple's WiTap sample code demonstrates how to use Bonjour to network two iPhones together.
I devoted a section of the class I teach on iPhone development to Bonjour networking, for which the course notes can be downloaded in VoodooPad format here (go to the Networking class for coverage of this).
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