Daniel and realityinbox are right: before you return the lens, make sure you focused properly. It's not easy. I usually take 10 pictures and keep the sharpest. I use liveview at 10x exactly at the part of the picture I'm evaluating. (If you focus the center, the edges will be out of focus because the camera is probably not square to your target).


Even the 70-200 II is not perfectly sharp in the corners. Here is a crop from the lower right corner of mine at 200mm f/2.8 taken with my 5DII:


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.25.93/Mk-2-lr.jpg[/img]


It's a little tough to compare to the xsi because on one hand, unlike the pic from your rebel, it really is in the far corner of the 35mm frame. On the other hand, the pixel density on the 5DII isn't as high as that of the rebel, so your pic reveals more softness.


Another thing to keep in mind: not all corners are created equal. My 70-200 has one corner (upper left) which is softer than the other three:


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.25.93/Mk-2-ul.jpg[/img]


I actually considered returning mine, but in the end decided against it: the "bad" corner is still better than my Mk1's best corner, and if I returned my lens, maybe I'd get one with 2 bad corners. I just have no idea how common that is.


By the way, many people report that the 70-200 II is as sharp as primes such as the 135mm f/2. Mine isn't quite as sharp as my 135 f/2 @ f/2.8, but the good corner is close. Here is crop from the corner of my 135 f/2 taken @ f/2.8:


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.25.93/135-lr.jpg[/img]


I won't tell you if you should return your lens, but I hope some of this info is helpful.