Mobile site is a great idea, but using a subdomain is not that smart.
Because then you always have the annoying situation that computer users click on a link and see the mobile site and the other way round.
If a mobile device is detected it should redirect to the url with an additional parameter that brings you to the mobile site.
Or better: a site that changes layout depending on the screen/window size (#responsive design). Like
crunchbang.org or synology.com.
That that would not work with default punbb i think. The crunchbang community uses fluxbb a fork of punbb.