I'd prefer to see style of the month than board of the month as vB.com already has that.
(my own view too)
Actually what vb.com has is a Style of the month, well they mostly base theirself on styles anyways, for this we could more focus on overall effort put in and so on...
With UBB, it was the modding community staff who chose the winners. The staff (admin, moderators, support) consisted of coders and designers. If this is to be an official contest then the community staff should be the ones to judge or at least play a larger role in the process.
Another way of judging is by peers. Everyone has their preference and it gives general users a voice. Though, there are problems with peer voting such as knowing and understanding how much work entrants put into their forum/site design.
Along with the judging criteria: Design, originality and content, another criterion should be integration. How well did the web-designer integrate their forum with the overall site.