No, not at the moment there isn't. I did briefly experiment with one a couple of years back but there are 2 problems for me with a mobile skin.
It has to be maintained. Every release of vBulletin means updating quite a few templates. I now have one set of templates but this would double the number of templates to maintain (ie. double the work for me at each release!)
The income to run the site comes from displaying the adverts. Mobile skin = no adverts = no revenue