[eluser]vokic[/eluser]
Well I see few possible solutions here:
1. Use additional field in db for thread which will hold user ids who read the thread and when you display just check user id against that field. It might get big but if you make it so that every time a new response is posted, that field is erased and a thread is marked as new again... Something like this:
12 456 45 88 etc..
2. Use a text file for the same purpose
As you think about it you will most likely show between 10 and 20 threads per page which means 10-20 text files which is really not that much.. Unless of course those threads will be seen 1 million times each hour or so.. In that case, throw in the adsense and open up for a decent hardware
Forgot to mention for the first one that I think is how vbulletin actually works or some kind of hack for it or something and that I've read about it in the past... If I dig up a link I'll post it...
