abushahin - 8:40 pm on Nov 28, 2011 (gmt 0)

The recommended media type for serving XHTML is application/xhtml+xml. More browsers are likely to support that media type, and optimize the rendering for it. application/vnd.wap.xhtml+xml is widely used and supported in mobile networks as well but is unlikely to be supported by non-mobile browsers.

from w3c website
ps they have a mobile site validator