The current Titanium SDK (1.8.2) doesn’t support Xcode 4.3, as Xcode 4.3 has a totally different structure in terms of tools and utilities. Until Ti 2.0 comes out, here is what you can do to get it working:

1) Follow the instructions on the TI web site
http://developer.appcelerator.com/blog/2012/03/titanium-and-xcode-4-3-revisited.html#cheatsheet

2) This might not be needed on CI builds, but was on my release build:
Open TI Studio, go to Preferences >Titanium Studio > Titanium – and hit the REFRESH button on the IOS SDK home (it should update to the Xcode 4.3 path in Applications)

3) I cant ever change the IOS Simulator settings from within TI, it always crashes, so I usually open the IOS SIM myself, change the device, and then launch from TI. So to select the Ipad Retina, launch the NEW IOS Sim from here:

open /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/iOS Simulator.app

And then go to Devices > ipad (retina)

Now you should be all set.

As I often use spotlight to launch the IOS sim outside of Ti, I created a launcher app for the new Sim (A Symbolic link doesn’t work as spotlight thinks its a system file).

Download IOS Sim 5.1 launcher
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UPDATE
I initially upgraded Xcode and kept my 4.2 install, but this caused all manner of issues after a few hours, and TI couldn’t compile – kept looking in the wrong Xcode dir for some libs.

Installing on a machine with JUST 4.3 works fine though.