http://tech.slashdot.org/tech/08/03/24/1959201.shtml 

an interesting comment on slashdot - indicating that will crawl urls seen in gmail emails.

"Nothing, absolutely nothing, stops Google from harvesting everything they can get their hands on- and they have the storage systems and human expertise to do it.

Case and point: I emailed a link to a wiki I had just set up to 3 people, two of whom had Gmail accounts. A spider from Google hit the page hours before anyone else did, hitting the wiki just after I emailed the link out. There were no public links to the site, and no referral URL."