I've got a site that was running WPMU 18.104.22.168 that was upgraded to 3.1.2. For some reason, if you go to a permalink and leave off the trailing slash it redirects to the right page but goes to https. I can't see any differences between it and others that I've freshly installed 3.1.2 (and .3) on that work fine.
Luckily I still have access to the old 22.214.171.124 site (different hardware) and I can see that the behavior is correct on the old site as well. So I (or the upgrade?) mucked something up.
I've done some googling and checked the support forums but haven't found anything that covers this particular issue.
I've compared the .htaccess file between a site that works normally and this one (note, multisite is enabled) match exactly. And there's nothing particularly funky going on in the .htaccess, it basically matches the multisite setup with some secfilter options at the bottom.
I've doublechecked siteurl entries in the database and the wp-config file but I can't find https being specified anywhere.
I can see quite specifically via firebug that I get a 301 redirect. On the sites that work correctly it bounces back to http://sitewhatever.com/correctpermalink/. On the bad one, it bounces to https but the rest of the permalink is correct. Of course if you just enter http://sitewhatever.com/correctpermalink/ it works normally.
Any ideas what to look at or try next? It's a live site, so I have to be kind of careful as to what kind of debugging options I turn on unless it's during low usage times. But everything I see points to wordpress internal redirects and not anything being done by apache.