For my app, QuickPic, I needed to show the user the IP address of their iPhone so they could type in the URL to the browser. The iPhone SDK provided no simple way to get the IP Address for the wifi connection. There are some undocumented methods that work ([NSHost addresses]), but I didn't want to risk them pulling that out of there and my app breaking. So I wrote some C code (cobbled together from various sources) that will loop through the network interfaces and retrieve the IP address.
Here's an Objective-C method to retrieve the IP address of the wifi connection as a NSString.
Update (7/10/09): if this isn't working for you you may need to include the following C headers in the top of your class implementation as well
Note: this code will work with the Simulator as well though the interface may not be en0. The iPhone Simulator seems to just use the underlying active Mac OS X network interface. On my macbook pro using wifi, this is en1, but your mileage may vary.
This will also work in Mac OS X since the iPhone OS and Mac OS X both use a lot of the same unix underpinnings.
This code is released under the MIT license.