Although.... we could cheat.

My typical usage of the site is to find the new posts on Recently Active Threads and then click through to them (or their parents).

We could do the same as Holli suggested for RAT/NN and, if the user has a "time of last visit" set, add a class to the posts on that page that are newer.

It clearly wouldn't work for older threads found on (eg) Seekers of Perl Wisdom, but it wouldn't hurt either. For common usage, it would be fine. And cheap.

Clint