An XHMTL MP document can use a WAP CSS (WCSS) style sheet. You can detect the usere agent via either javascript or code to determine the appropriate style sheet.
The only tricky part is that the WCSS file will get retrieved after the XHMTL MP document and should be kept small and simple to expedite the display. The WCSS file will not be kept in the browser cache and will get downloaded for each page.
EDIT: You may want to check out:
http://www.developershome.com/wap/wcss/
Edited by dpyers - 01 February 2011 at 3:23pm