CI Site not loading

[eluser]CodeIgniterNewbie[/eluser]
I have a peculiar problem with my CI app. On one of my servers, it runs OK. When installed on another server, I can't load any controllers. DB, base URL, etc. are all configured properly from what I can tell.

I installed Live HTTP headers plugin for FF. On the problem server, when I go to www.domain.com/index.php/, I get the CI 404 page (and Live HTTP headers reports this properly). When I go to www.domain.com/index.php/controller_name/, I get nothing (and nothing from Live HTTP headers).

Any ideas what I should look at?