Originally Posted by keller
Are you certain that the IS was turned on? Your shutter speed (1/125th) is a little slow for handholding without IS (you'd want at least 1/200th). Bryan's review says the lens should be good for three stops of stabilization (even though marketed as four stops), so you'd want 8/200 or 1/25th so you should have been safe here if IS was on.
I'd try a few shots over a range of apertures. If smaller apertures appear more in focus, it could be a DOF issue or it could be a focus error issue, so I'd then try using a focus chart to see how your lens is doing. I'd also shoot some on a tripod, just to isolate some issues.