I've racked my brains over this one, and I'm probably just making a stupid mistake. After switching to Xcode 4.5.1 (it may have happened at 4.5) I simply cannot load an app into the iPad Retina simulator. I've edited the compilation scheme to say to always use the Retina display for the iPad. No love. It used to be that you could change simulators on the fly and whatever one was open would be the one the project compiled for. But I can't figure out how to do this in 4.5. Help!
Well, after a long session of "find all of my settings files and save them, delete Xcode and all of its settings files, reinstall Xcode, re-download simulators and dockets, replace settings files, etc., etc." I found myself in the same position. No Retina simulator for me.
The problem wasn't Xcode or the project file--the 5.0 simulator will simply not launch in Retina mode. The 5.1 and 6.0 simulators work fine, as I discovered when I switched to them in the compilation scheme. Switch back to 5.0 and I'm stuck in normal simulator mode. The reason I've been using the 5.0 simulator was to test lowest-common-denominator iOS features. That, and there is a bug in iOS 6.0 that affects large packet transfer via IPSec that hasn't been fixed yet.
So, I still don't know what the missing Derived Data status was all about, except that it's fixed now and I assume that reinstalling the app was the fix.