Couldn't find iOS SDK 5 in Xcode
Sometimes I spend hours banging my head over dumb problems. I needed some iOS 5 functionality (the built-in JSON serialization) and had the latest version of XCode installed on my Mac. So said the App Store anyway. But in XCode the latest SDK available was 4.3.
It took me a while, too long, to figure out that I'd locked a previous version of XCode to my dock. And that previous minor versions of XCode were each standalone applications, so moving from 4.1 to 4.2 meant you had to uninstall 4.1.
The solution was to delete the link from my dock and then to Spotlight search for all the available XCode's on my Mac. The newest version did provide an option to delete older copies on the hard drive.