[eluser]stuffradio[/eluser]
A clunky way I can think of for seeing if you read the post or not, is to have a LONGTEXT field in the db for each post that has userid_read. You could use explode and if the userid is there, the thread was read.
Note: I said this was a clunky way to do it, and only effective until you have over 100-500 users I would think.