Well I personally think it would be a really nice addition to the site, since it's a bit of a hole in the whole picture of cultivation, other forums acknowledge greenhouse growing as totally separate from other styles, and naturally I have my own selfish reasons, which is the greenhouse I'm putting up this weekend..
It's up to the site admin to decide how valuable the sub-forum would be.
We already have plenty of historical posts about greenhouses, and I'd volunteer to go sift through and move them to a new forum if it is created to make them easier for GH cultivators to find and use.