Mine's PP2012 SR2. I've downloaded SR2.1 but haven't installed it yet, so can't say yet whether this will still happen after. At the moment though I'm rendering images at 2400x3000px with a background image of exactly the same dimensions and I get a small gap of about 20px down the right hand side (occasionally along the top instead). It's not a big deal, I just have to remember to crop the render afterwards.

Ground plane is always off btw.