I created this community wiki to bind together a number of other Q/A's I've seen here on SO. If you're here, you've probably installed the final version of Xcode 3.2.3 with iPhone SDK 4 ( download link , reg reqd), and then encountered one or both of these problems:
In your previously working xCode iPhone project, you get "Base SDK Missing" somewhere. Something like this:
In a related but more general observation, none of the previously available panoply of Device or Simulator SDK choices (e.g. 3.0, 3.1, 3.1.3, ...) is available where you used to see them.