Though it's been brought up and discussed at length multiple times already, the main barrier in making something like this happen is that it actually tends to make the site alot less usable with no real benefit.
If you have a mobile device that's geared towards using the internet at all, like an iPhone or the better Android based phones, if your connection is good, the site loads and browses just fine. You can't watch youTube vids inline on the iPhone, but you couldn't do that with a mobile skin either. If you have a mobile device that's NOT geared towards using the internet, like the less 'smart' smart phones that cost a bit less, then making a mobile version of the site doesn't actually make it any more usable - your phone's just not designed to display the amount of information found on the typical internet site, and the only way to cater to that is to actually make contented targeted at those limitations.
And I write this as someone that uses and writes on this site 80%+ of the time on his iPhone in good old mobile Safari, on the front and admin ends (this post included). I even made a mobile version of the site already and played around with it for awhile way back when the idea was fresh, and all it did was make the site less usable while giving me one more thing to debug and develop