Giving back to the community

You only have to provide source/give back to the community if you make alterations to existing GPL'ed code (unless I am very mistaken).

If Google use out-of-the-box Linux configurations but have their own separate software which runs on that, they can license their *own* code any way they please, or lock it in a vault under 20 feet of concrete, protected by hungry tigers and killer bees if that's what lights their candle! They'd only have to feed back any alterations they made to the GPL'ed bits of the system.

Seems like they give a lot to the community, look at Summer of Code for example.