Ran into this one today, got the following exception dealing with Facebook's Three20 library:
2011-02-23 15:37:10.964 HeyZap[49847:207] -[UIWindow findFirstResponderInView:]: unrecognized selector sent to instance 0x603f9d0 2011-02-23 15:37:10.968 HeyZap[49847:207] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[UIWindow findFirstResponderInView:]: unrecognized selector sent to instance 0x603f9d0'
Turned out, we forgot to add the Linker flags necessary for libraries like Three20:
-ObjC -all_load
Slapped those two into the "Other Linker Flags" array in Build Settings -> All, and poof, our error went away. Now for more coding...