Making the forum work for a mobile device will require a new style specifically for said devices, one designed to cut out the extra junk and display the absolute minimum of content in a format a small screen can manage. Most smartphones can view regular web pages (the iPhone is one I am personally aware can) without too much difficulty, although the amount of scrolling required to read anything will quickly make one want to tear one's hair out.
Could it be done? Of course, and there are many examples of sites that do it. Google is one. However, it would take a tremendous amount of effort (and a developer with a smartphone to test the thing as it was made) to implement. Despite what that link the Jeffman posted may indicate, it isn't a simple thing to do if it is to be done well.