I don't think there is any planned expansion info available. You'd have to phone them and ask about it, but the time frame will not be easy to get out of them, and if it is, its wrong.
Oh, besides on their coverage maps of course. You can toggle between current and planned coverage on the maps. That might help, but the planned basestations have no time frame, be prepared to wait.
EDIT: Nope, the new maps no longer have a planned coverage option. All you can do is phone..