I can't go into huge detail, since I haven't implemented it in a 'tour de force' fashion as of yet, but if you read the following links, it shoudl help:
http://www.phpbuilder.com/columns/tim20000526.php3
(the <Location /local> in this script doesn't appear to work for me (using php4) even though I have control of the server - don't know why - use <Files) instead)
http://www.zend.com/zend/spotlight/searchengine.php
http://www.webmasterbase.com/article/485
They all pretty much say the same thing -
3 levels -
apache control
htaccess control
parsed control
Which you pick is up to you! If there are any more examples, then please post here - I think it's an interesting concept.