Edit Project Settings -> In the build tab -> For Mac OS X Deployment Target, change it to 10.5 for XCode 3 (not 10.6 even if on 10.6) and see if that helps.
For XCode 4, you'll need to set it to 10.6
P.S. Make sure you set that for all targets, not just release or debug. (if you didn't, one would fail, the other wouldn't)
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