PCB doesn't do ground planes the same way as other packages, we actually draw the copper, and anti-draw the reliefs (negative layers have you draw the reliefs). Planes are no different than any other polygon on any other layer (you created your plan by drawing a big rectangle, yes?), so if they can do regular layers, PCB planes should work too.
In theory, if they ask "which layers are planes?" you can answer "none" and it should still give you what you expect. Having said that, it's probably a good idea to send them the gerbers with a note saying that we do planes different, and please check to make sure they'll come out right. When you find out, let us know, and we'll either (1) fix it if we can, or (2) note that we do the right thing ;-) We do, however, use negative plots for solder masks.
