A few thoughts:
1. Google has admitted to sharing data between the AdSense MediaBot and GoogleBot. They can find pages which contain AdSense, even it there are no inbound links.
2. The Google Toolbar sends data back to Google. If anyone running the Google Toolbar visits your page, Google learns about your page.
3. WordPress has an installation option asking whether you want your site public or private. I wonder what all that does?
4. If you made a post to the blog, WordPress might have promoted it by pinging one of the Update Services.