I loaded a project in Xcode that I hadn't used in a while. "Base SDK Missing" appears in the upper left drop-down menu. I tried to build it anyway, but got the "Build Failed" error, that "error: There is no SDK with the name or path 'iphoneos3.1.3'".
This is with Xcode 3.2.6.
I re-installed Xcode, and I had more problems. Then I re-installed it again, and it worked. Odd.