Basically, it would be more like subcategories rather than subforums. I think this would perhaps be a decent addition.
Well I think like this. For example you have a gallery forum.
In the gallery forum you would never have a post. But have a gallery sub-forum for each year.
So name the gallery forum xmas.
Then have Gallery sub-forums
xmas 2009
xmas 2008
xmas 2007
So you would never have a image posted in the main forum only in the sub-forums
Of course I think this should apply to all types of forums not just a gallery forum.