Forum Moderators: open
Perhaps someone has posted a link on their site to your site using https instead of http - did you try looking at backlinks?
Check your DMOZ and Yahoo! listings - if they have your site listed as https - Google will index you as https
Https rankings are also weird - I've seen greybar https pages outrank high PR pages?
If you don't want https pages indexed - robots.txt or 'noindex,nofollow'