Hey, as I know.... This is a good one.. BUT
it will add too much data that is not really needed to the database..
Why don't you make an Additional Table .. let's call it (thread_read)
and add 3 fields (userid, threadid, timestamp)
and then add to it so you can at least delete them from time to time especially if the topics are too old --> no need to specify who read it
I hope this would help
thanx