There are no plans for adding forums for other brands (or any other subject areas) under the existing vBulletin software. Under the software Pekka's developing, it will be easier for both Moderators and members to deal with a potential explosion of forum areas. When will that be? When it's right, not when it's expedient.
However, even then, the metric for creation of new subject matter areas will still be the level of interest shown by members. We've had requests for new forums on birds, fashion, food, motorsports, underwater and weddings among others. There was a clear need for Birds (which was dominating Wildlife), Motorsports (which, in the season, swamped Transportation) and Weddings (ditto for People); the demand for food, fashion and underwater hasn't been anywhere near that level, so they remain part of whatever the photographer may think they most closely fit into.
For the time being, there is a thread set up for Nikon discussions. The General Photography Talk area would be appropriate for new threads on M4/3 or other camera brands as well. If you decide that you want to demonstrate the demand, I suggest that you start a single thread for each relevant area, rather than creating a new thread every time someone has a new question or a manufacturer releases a new body or lens. It'll make it easier for you to find relevant information and for us to gauge the interest.