The much-noted green background behind John McCain during part of his speech was in fact the verdant lawn in front of an imposing edifice, as you can see in this wide shot. One of McCain's many dwellings? No, it turns out it's the Walter Reed Middle School in North Hollywood, California.
What, you ask, was it doing as a backdrop for McCain's speech? No one seems to know for certain, but the current speculation making the rounds is that someone on the McCain team intended for him to be standing in front of a projection of the Walter Reed Army Medical Center, which would make a great deal more sense, but goofed and grabbed this photo instead. (Alternatively, the McCain camp may be addicted to that particular electric green and the middle school's lawn provided the best example they could find that was not on a 60s-era album cover.)
TPM is on the case and trying to confirm.