>Would it be possible to store by-thread read marks client side (by
>cookie, or something similar)...then, if al lthe threads in a forum
>are up-to-date, mark the forum read as in the current system?

Not in any reliable way. The cookie spec limits a client to a maximum of 20 cookies per site. And there is also a limit on the maximum number of bytes a client can store per *site* - total of all cookies. There is also a limit per cookie. Storing timestamps for every thread could add up to a lot of information to be storing. And, of course, it locks you to that browser on that machine. If you went from work to home, or school to home, etc, all of your marks would be gone - everything would be unread. If you use NS4 with roaming it might be ok, but that's not likely these days.

One of the big advantages to a web forum, over say news, is that it is client independent.