It can throw 500, mouse, and often does. htaccess loves to throw 500 errors around, one typo can break your entire site. If mod_rewrite is not enabled, I've seen instances of it throwing a 500 error and instances of it simply not processing and dying silently unless you are paranoid enough to check your error logs.
Speaking of error logs, Carla? Do you have access to yours and if so, is there any more information you can give us from that?