Is the word frequency a configurable option? It is there to stop the search hanging on potentially expensive search operations (e.g. searching for "the"), but the database/code would have to be very inefficient to struggle with some of the words it filters. It really bugs me too, I come up against it a lot. Sometimes you can figure out a different way to search, like a phrase in double quotes, but quite often not.
Let's be honest, the phbBB search isn't particularly well implemented: it filters these words even when constrained to vastly smaller sets (e.g. searching the title, or using several words together). Though to be fair, it's not easy to implement good search, it's just we come to expect it because of Google. Sometimes it's best to let Google do what Google does best:
http://lmgtfy.com/?q=site%3Ataccmd.tact ... es.net+tau
