I think they already did (kind of). As far as I can tell from a quick glance, this is Django on top of Google's infrastructure. But they aren't about to open-source their infrastructure.
But they aren't about to open-source their infrastructure.
Wouldn't matter if they did -- you can't open-source a bunch of contracts with trucking companies, nor would any sane company want to open-source their testing routines for the hardware they build in house to manage the mess. Parts of the infrastructure are, however, published as patents :-)
Google's operational efficiency in deploying new capacity is the crushing advantage they possess over 99.999% of the companies on the planet.
That would be totally awesome, and you could make the case that it benefits them to have their App Engine platform become a more standardized/open framework!