Thanks to everyone who posted their trace details here (and those who emailed me directly). Unfortunately I still don't have enough detail to diagnose a common cause (or common causes), so I'd like to request a further favour. Can people repeat the earlier exercise of capturing session details via fiddler, except this time, when they get logged out, press F12 to stop capturing traffic. Then click in the http sessions pane, and go to Edit->Select All (to select all the session details). Next go to file->Save->Everything to zip. If you can then send me copies of these zip files to cathal.connolly(at)dotnetnuke.com , I have a better chance of diagnosing the problem.

Please note, that whilst Dotnetnuke encrypt's your cookies, which means they will not reveal any pertinent details to me, in theory I could mock up new cookie files and use them to access your portals, I hope the fact that I'm a coreteam member will mean some of you will be good enough to trust me, as I would never do anything like that. For the really paranoid amongst you, create a brand new user in your portal, capture all the details, and then delete the user, as the cookie details will no longer have that user to authenticate against.

Thanks,

Cathal