I'm not sure if I've blogged this before, but I've been using this method for using CSS files in Windows Mobile IE. My method:

1) Create the link for the handheld style sheet using media="handheld".

<link rel="stylesheet" type="text/css" media="handheld" href="css/handheld.css" />

2) Hide your default big-browser stylesheet using media="Screen" (with the capital S). This hack hides to Windows Mobile IE for some reason.

<link rel="stylesheet" type="text/css" media="Screen" href="css/master.css" />

This method works for me. Good handhelds and phones will recognize media="handheld", but not sure all do. I'm always looking for other suggestions to get decent and readable results in handhelds, but only test in PPC and my Motorola phone. I have not used phone emulators in a while, but I assume they're still out there.