If you see a message like “The version of iPhone OS on (your phone’s name) does not match any of the versions of iPhone OS supported for development with this copy of Xcode” after the 3.0.1 update, the quick fix is to run in a shell:

sudo ln -sf 3.0 (7A341) /Developer/Platforms/iPhoneOS.platform/DeviceSupport/3.0.1

And then restart Xcode and reconnect the device. A long discussion about this at technolosophy.