Googlebot watches Gmail. I recently bought a new domain name and kept an active search window open for very specific keywords to make sure it didn't go live until I was ready for it to. The people who received the link to the domain are trustworthy to not blab about it (e.g. tweet, post, etc) until I'm ready.
That said, two days ago I sent a link to precisely one person with a Gmail account. This morning, Google search results returned positive for search discovery. Googlebot is definitively watching Gmail for references to new domains and the number of references passed around within Gmail might actually affect PageRank.
This shouldn't really surprise anyone, but it should be something to be aware of if you want to keep a product launch a secret. In my case, it doesn't really matter.