W3C Mobile Best Practices Working Group
http://www.w3.org/blog/BPWG/2005/11/14/to_wml_or_not
XHTML-MP tutorial
http://www.developershome.com/wap/xhtmlmp/
Summary of W3C document types
http://www.w3.org/2005/MWI/BPWG/techs/DocumentTypes
CSS Mobile Profile
http://www.w3.org/TR/2006/WD-css-mobile-20061208/
Comparison of XHTML-MP and XHTML Basic and others
http://pc.dev.mobi/?q=node/119
More comparisons
http://www.thewirelessfaq.com/what_is_the_difference_between_xhtml_mp_xhtml_basic_wml_imode_and_hdml