To use this effectively, you will need to modify the user agent string to spoof a mobile device that supports the required content type. For this, use Modify Headers (http://modifyheaders.mozdev.org/) or the User Agent Switcher (http://useragentswitcher.mozdev.org/).
One device I know that supports this content type is a Motorola E1000: MOT-E1000/80.3E.29. MIB/2.2.1 Profile/noMP
Example sites are: http://mobile.lastminute.com/, http://www.ebaymobile.co.uk/
If you create a public mobile site, submit the URL here: http://xhtmlmp.mozdev.org/bugs.html, and I'll add it to the list on the development site.
application/xhtml+xml: This is supported natively by Firefox. If you are seeing an XML node tree, the site developer has not added the xhtml namespace in the root node. See my blog for more info: http://www.garethhunt.com/index.php/2006/01/29/mobile-profile/
text/vnd.wap.wml: Use the WML Browser (http://wmlbrowser.mozdev.org/)