Just a heads up that (at least for me) the advisory from Apple detailing how to get Xcode to play nice with 3.0.1, didn’t work. This was because of some unintended spaces that were inserted as the line of code was broken up to fit on the page! This should be just one line of code that you paste into Terminal (expand the Terminal window to make sure). Here’s what worked for me:
ln -s /Developer/Platforms/iPhoneOS.platform/DeviceSupport/3.0\ \(7A341\) /Developer/Platforms/iPhoneOS.platform/DeviceSupport/3.0.1
Note that if you didn’t instal Xcode in the default /Developer location, you’ll have to change the directory in the code to reflect this.
The advisory PDF is at the bottom of the list of downloads in the Apple Dev Centre, BTW.