I've been getting this warning lately, following adding a new view controller, with its companion NIB file to an iPhone OS project built with Xcode 3.2.3.
While this warning could be legitimate, if someone removed an outlet definition from the source files, without updating the NIB, in this case it happened without me making any change. So the project would compile and run properly, then I'd add this controller, build and get this error.
Turns out this is a bug in Xcode, and it happens when you build the project while the app is still running in the simulator. Just quit the app, build and the warning goes away.