A question at the Dutch forum about detecting mobile browsers inspired me to create this snippet.
After installing the snippet, you can use the function isMobile() in your template, and decide on the result what .css you want to include.
The snippet is a wrapper around the detection script that is made by: Anthony Hand
The latest code (included in the snippet) can be found here:
http://code.google.com/p/mobileesp/I have setup a small website so you can see the snippet in action.
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http://m.ww3.nlDon't forget to go to the site with your mobile browser too 
A download (version 0.1-experimental) is also available on the same website.
The snippet will not tell you things like screensizes or if javascript can/cannot be used.
Future versions might give a bit more info about the device used (like isIphone, isAndroid, isWindowsMobile) but that will be the max for this snippet.
Have fun with it..
Ruud