Had a look at it now.
There is an alternative to cookie based thread markings, and that is database based thread markings. The database would then keep the "true" read/unread marks indefinitely, but this would also highly increase the processor load, which may slow things down.
So, at a first glance it does not look like a good idea. IF however, this is a feature more people find lacking from the forum, I'm willing to do a trial run to see how it goes. But as I have allready planned to add 2 more pieces of forum-integrated software to the site allready, I'm a bit sceptical to changes that might slow the site down.
SGL.