I seem to be having a very similar problem. I just upgraded to wordpress 3.0 and thesis 1.7 this afternoon. I was adding a new menu and moving some pages around in the navigation. Things were going OK for awhile, then I found that that two pages were automatically redirecting:
mysite.com/marketing is automatically going to mysite.com/services/marketing
mysite.com is automatically going to mysite.com/marketing-2
I have cleared cache and cookies and restarted firefox but it made no difference. Have deactivated all the plug-ins: no difference. Removed the code I'd added to custom_functions.php in Thesis to make it work with the 3.0 menus...no difference.
I had some menus built with the WP menu manager plug-in, and thought I'd try the new 3.0 menus, so I'd added the requisite code to Thesis and was moving things around, but decided it was easier to use what I'd already started. So I may have done something to confuse the navigation.
I'm taking it macmanx suggested something offline that's not in the discussion here? Please let me know if there's something I should be looking at in the database. I can stumble my way around phpmyadmin but don't really know what I'm looking for.