It all depends on how it's implemented to be honest. I doubt it will be easy to do for everyone, or else you would run into thousands of sub-boards flooding the area. But that of course begs the question of how these Boards will be created in the first place.
One guess is that players will not directly make sub-boards, but instead have it be somewhat automated. An interesting thought I had (looking over all the idea thrown out over the year) is creating Boards based off of tags. You create a post with a certain keyword as a tag. If that tag becomes popular enough (above a certain discussion/post rate threshold) a Board will be automatically created with that keyword.
Another (and probably more likely) guess takes Tamat's hint at a form of player moderation. Certain users that are active enough in certain Boards will have the power to create Boards as they see fit (maybe within a limit), and they have primary moderation and governing over said Board (of course red mods will probably check in now and again). If the Board remains popular and well-discussed then it'll stay, but if it becomes inactive due to lack of appropriate moderation or simply lack of discussion it'll time out. Of course the largest problem with this idea is it it's sitting right next to the idea behind sub-reddits and the massive GD lash-back will be.....ah, what's a good word....interesting.
And then there's the simple idea of votes. Someone could introduce an [New Board] Post with a unique voting option and if enough votes are reached the post will be created. This [New Board] idea could even be a Board in of itself, kind of like a Kickstarter for categories.
BUT this is all speculation with absolutely zero concrete information, I don't want to start any rumors now.