Every once in a while a borked Eclipse build gets nailed down to the absence of this file, with the simple fix of cleaning the build, or occasionally doing a deeper clean on the project's metadata. I can live with this state of affairs, but the curious side of me has to ask why.
A search for it on developer.android.com turns up nothing :(, and a search on Google turns up various threads by folks wondering why their build is broken.
This guy is the closest thing I've found to an explanation of the thing itself. Hopefully some nice, patient guru can shed some light on the fundamentals here without this noob digging too far into the source :)