I know nothing about phpBB, but lets say that you have board with 100.000 threads and 10.000 members. Then when some member will deside to "mark all threads read", vB will have to run query with 100.000 affected rows. And update of text field in 100.000 rows at once will kill your MySQL server, and if it will be still alive, what if 200 members at once will decide to do same thing? So I am sure that it will never be possible in that highlighting scheme. Besides i don't understand what for do you need it - sorry