I think Trev's tried a few times to get it right but it's proven elusive to perfect.
I have said many times the forum search is broken, and won't be fixed. The google powered search does a suffecient job, sometimes even better, the phpbb search was always week when it does work.
When the forum is upgraded to phpbb3 eventualy, the forum search will work again. The search in phpbb3 has been Greatly imporved.
I won't get into the technical reasons why the search doens't work, but there it should only be the case for a little while longer.