UPDATE (3/19/2012): Xcode 4.3 is now supported in our continuous integration builds. Go to https://build.appcelerator.net to get it, and make sure you are downloading from the master branch.
Apple has recently released Xcode 4.3, which some developers have already started adopting. Please note that per our compatibility matrix here, Xcode 4.2 is the latest Xcode version that can work with Titanium. We are working to add Xcode 4.3 support as soon as we can.
An unsupported and untested workaround can be found on the Q&A here, but your mileage will vary. Please refrain from upgrading your Xcode tools to version 4.3 until we can release tooling support for that version.
Thanks, and we’ll update this post as we have new information.