The default config file for Minidlna in Tomato USB does not work with an Xbox 360. There's a bit of background info here: http://gareth.halfacree.co.uk/2009/09/xbox-360-and-minidlna on the config changes needed to get the xbox 360 to work with minidlna.
The following lines in /etc/minidlna.conf need to be changed to allow the Xbox 360 to work. Obviously, the presentation url depends on the current ip address of the router.
To enable newbies to make this work easily, this config could be enabled via a tick-box on the Media Server configuration web page, similar to the existing "TiVo Support" function.