A few months ago I sent my 17-55 2.8 IS lens in to Canon service because it front focused dramatically at the wide end of the zoom range. When it came back, it back focused at the narrow end of the zoom range.
I sent it in again and they had me send in my 7D as well.
When the camera and lens came back together, they now front focused at the wide end of the zoom range, much like they did in the beginning.
I did some more extensive testing putting that lens on different bodies and using the 7D with different lenses from my collection. The body had similar problems with other zoom lenses, but in different parts of the zoom range! Oddly, prime lenses still worked perfectly.
The 17-55 lens showed wild focus problems on my 40D body, I think still front focused at the wide end. (Don't have my notes with me now.)
So I speculate that Canon service attempted to adjust the programming in the body to make it work with a defective lens. I really don't know if that is correct. I have not been able to find a full explanation of how the focus algorithms are supposed to work, when they are working correctly.
The whole focus system sucks if you ask me.
Right now the lens and the 7D are back at Canon service for a third attempt. I sent them to New Jersey this time instead of California.
have you read this: http://www.canonrumors.com/tech-articles/this-lens-is-soft-and-other-myths/ ?