Honestly pretty damn secretive. I worked at Google for the summer and that was the only building on the campus I couldn't go into (I could even go right up to Larry's office door but not to X). My good friend was an intern on X and he never told me a word about what he worked on or who he worked with. That's just an intern. So even though they do reveal some projects for PR once in a while, it's definitely a very secretive part of an otherwise fairly transparent company.

Same page as the one any secretive organization uses, don't tell anyone anything unless you need a PR boost then trot out some program that meets the need. Has worked for IBM in the past, works for Google, heck it even works for the CIA.