Needing a license compatible implementation of UPnP I could use to locate MythTV frontends, I made the mistake of using the C++ version of CyberGarage's library. This was a mistake as the C++ version (1.7) is old and filled with bugs and leaks. The proper version to use is the C version (2.2) which is much less buggy and even comes with an Objective C wrapper class.
If you are writing an iPhone app and need to locate a UPnP device, this is probably the way to go.
Oh, and one thing to consider is that if you look through the code, you will see that the library avoids opening up connections with network interfaces that have the IFF_LOOPBACK flag set—for obvious reasons. You might want to also avoid network interfaces with the IFF_POINTOPOINT flag set, as that is as like as not the cell network radio, and you probably don't want to make UPnP inquiries over the cell network.
[Update: Looks like TCM Port Mapper might be the preferred library, but I haven't tried it.