For some reason, I could not get .htaccess files to work on my system for the longest time. I’ve since upgraded to Leopard, and I eventually just gave up on it (I could do that, since I’m only running a development environment).
Anyway, the solution depends on which version you’re running:
For Tiger, edit /private/etc/httpd.conf AND /private/etc/users/username.conf for the correct directives
For Leopard, edit just /private/etc/apache2/httpd.conf
Days of pulling hair solved because I was looking in the wrong places.
Update: because of the upgrade from Tiger to Leopard, ALL of the above directories were present on my system, but only /private/etc/apache2 was being used. This is because Leopard uses apache2 and Tiger uses apache 1.3. Obviously, apache2 uses /private/etc/apache2 instead of /private/etc/httpd. If you had installed Leopard from scratch, this would not be a problem.