Something very similar happened to me four or five years ago. I had just sat down at my laptop and opened the browser. Imagine my surprise when it correctly navigated to iGoogle, but under someone else's account. I looked around just enough to verify that I was locked into a session and then took some screenshots, wrote a detailed explanation of the past five minutes, and sent them along to Google. Never heard back.

The zip code in the iGoogle page weather widget suggested it was someone that lived in my general area, but I didn't recognize the name. This wasn't a shared laptop. I believe this also predated SSL-on-by-default so I guess it is possible traffic got mixed up somewhere.