With this code:
<link rel="stylesheet" type="text/css" href="media/css/mobile.css" media="handheld"/>
<link rel="stylesheet" type="text/css" href="media/css/screen.css" media="screen"/>
on my N78 the nokia's default browser and opera mini load screen.css instead of mobile.css.
What am I missing?
Nokia N78 uses the S60 browser, which reads only "screen" stylesheets. It won't read the "handheld". It doesn't support media query. Instead of trying to type everything here, have a look at this article and you will solve the problem - http://www.alistapart.com/articles/return-of-the-mobile-stylesheet
Using the iPhone as an example:
<link media="only screen and (max-device-width: 480px)"
href="iPhone.css" type="text/css" rel="stylesheet" />
I have no idea which browsers/devices this does and does not work for. The only mobile device I've developed for is the iPhone. But I'm sure you can give it a shot with your nokia.
Opera Mini uses to the media type screen because it fits its behavior and capabilities best. However, it will use handheld stylesheets if you switch to mobile view mode. See A developer’s look at Opera Mini 5 beta 2 for more information.