I have the same problem, it appears that you also made the jump with the new Xcode 4 upgrade and this appears to be a code incompatibility.
If you want to keep it for IOS (Iphone / Imac ) edit Scheme [Product/Edit Scheme/ Build/ build => Destination drop down list.
Make sure you have installed the Ios SDK before running Xcode.
Elsewhere if you have to compile the same app for the Mac, I'd like also to know the answer as this generate the same errors as you.
Deployment target is missing for specified SDK ...
Choose other "Deployment Target" (in the Build settings) and simulator will appear.
Note:It's happening when use 5.1 SDK(latest) with XCode 4 on Snow Leopard..
Tried the rest of these with no joy.
AFIAK this is a version control problem, in general not just a Git problem!
I gave a colleague a copy of a project that had modified files in it and this problem occurred.
However when I committed/updated the repo and gave him fresh copy.
This problem was fixed!