Unfortunately I don't know if that will ever be possible. Ok, let me rephrase that. It's definitely possible from a coding standpoint, but I don't know if that will ever be possible without making things real slow.
Basically to do this you'd need to have one more table that has 2 rows. A Userid and a postnumber. Every time someone reads a post/thread all of the post numbers would need to be inserted into that table. When the postlist page is displayed it would need to grab all of the post numbers from the thread and then do a query against the new table and the user's userid. Same thing with showflat and showthreaded.
When I first started coding .threads I was actually doing this, but as soon as the length of some of the threads started to grow it became apparent that it wasn't going to be fast enough.
