Well... I think the answer is simpler than it looks. I would think that revenue from Ad Sense, etc. is all secondary to this project. My thought would be that Google wants to create an OS that it will distribute for free, on the basis that unused CPU cycles are "donated" back to Google to use for its own internal processes (ie. indexing, crawling, etc.). Think about it, Google would have the worlds largest distributed parallel computer crawling and indexing away at their command all for a cost of near peanuts to what it would cost to build by purchasing all the necessary hardware.