100% agree with AlexDawson.
No need for XHTML MP. There are hardly any mobile phones that don't support HTML/XHTML.
I just use ?site=mobile or something like that to display a mobile stylesheet and the occasional bit of server side code that organises things differently.
When you design your main site, also design the mobile one. It'll take 10% longer and that's it. You use all the same pages and don't really need to worry about making a mobile site look too snazzy - it's so easy to do a mobile site.
Also, I test all devices and browsers (desktops and mobiles) on both my sites. So an Android phone can easily view my main site and mobile site to perfection.
There are some mobiles that I wouldn't even bother testing a full site on (Blackberry and IE Mobile) - just divert these with server side code to your mobile site = most others should get a choice.
There are some real issues with some JS/CSS commands though in some browsers but you can easily design a basic mobile site with hardly any JS and reduced CSS.
Full site = all the CSS/JS you want, designed minimum 960 wide.
Mobile site = everything basic with slightly reduced content using % so it fits the screen.
IE Mobile is absolute crap. Blackberry (not the new WebKit one) has a rubbish browser. I'm not a fan of Bolt or SkyFire either but iPhone and Android are excellent.
I've gone off topic but I think I've made my point.