Yeah, it could be apply the Powershot lens correction to a non-Powershot. (BTW, chenoameg are you taking notes? You can almost certainly do this kind of thing too.)

I seem to recall it merges left to right, which is probably why, while the left most window edge matches well on the grid merge, and the vertical lines get progressively worse as you move to the left. If you do a grid again, I'd reccommened standing on a chair for the top row and then sitting on the floor for the bottom row to eliminate a lot of the curvature issues. (You get less distortion is the plane of the walls is parallel to the place of the film, or in this case, CCD.)

But it certainly beats trying to merge them all in Photoshop by hand, time-wise.