back focus problem

I have 30d and 28-135mm
My pictures are coming out soft.
So I did the focus test. And it turns out it's back focusing.
does this problem usually the lens or the body?

I did it with 2 lens and only the 28-135mm wide open is having the problem...

another question, how long does it take canon to fix it, if I would come in to the service center?

If it's only showing up with one lens (and based on more than one test shot of each lens, defocusing between them), it's the lens.

How fast? Where are you? The US facilities typically have turned my gear (and most others I've heard from) in about a week.

Thanks for the reply.
I'm about 20min from the irvine center. I went there a couple of weeks ago for sensor cleaning and it takes them about 1 hour... I'm just wondering how long is it for them to fix back focusing, if I go there and wait at a nearby mall...
I'll be doing a practice shoot this saturday. I'm just wondering if I could have the lens ready by then...

I doubt, unless it's a quiet day, that they'll take care of that on a "while you wait" basis.