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There is an open source project on Sourcefourge that will do this called CyberLink for C
It is written in C which integrates with Objective-C easily, but they provide Objective-C wrappers also. With it you can create UPnP applications easily with Objective-C on Mac OS X.
This is not to be confused with the Cyberlink DVD player software. They are two completely different things.
There is also MiniUPNP, http://miniupnp.free.fr/ which is open source. It is in C and written for POSIX, no idea if that works on the iphone.