[eluser]kurucu[/eluser]
One weekend hiking in the Pyranese and I miss a great debate on my current problem! I've spent the last week pondering this exact problem with myself, and have spent most of that going round in circles.

My current thinking is a table with user_id, thread_id and post_id. Anything without an entry is unread, anything with greater post_ids has unread posts within it. The problem is size and complexity, as Jedd pointed out early on, for very vast forums. I'm not sure what would be presented to guests... perhaps "hot topics" with recent replies.

vBulletin, which actually I administer through another site, allows one of three modes to be selected: cookie time only (threads since last visit), simple DB (threads since last visit, infallible) and fine-grained DB (complete message read tracking). I've not looked at the code for the latter, but generally find vB code to be completely obscure, as they have a unique insight into the meanings of MySQL column types.

Having read all of your responses, I've still not idea what to do, and might just post a note on the forum suggesting that users keep note of read threads themselves! Being serious, my present thinking is implement the user/thread/post solution, and then see how it goes...