Recently, I switched to Xcode4 and when I compile my project I got following error.
ld: warning: ignoring file /Users/myname/Library/Developer/Xcode/DerivedD
Had the same problem. But the reason was different I suppose. I was using the library.a file built for simulator and trying to run it on device.. came to know that i need different .a files for simulator and device. I hope this helps someone ;)