Most likely you need to do an AF microadjustment in camera.

To test to see if the lens really isn't sharp and it is the lens, set it up on a tripod, use live view and manual focus at 10x magnifcation. Then check the results on the computer. It should be sharp.

There is a new program that will help you do the adjustment in camera, some have used it on the forums and seem to like it. I have yet to buy it myself. The link is here;

http://www.reikan.co.uk/focal/

Or you could send the lens back and try another copy.