Autofocus problems with the 17-55mm f2.8 IS lens used with the Rebel XT (EOS 350D)

My 17-55mm f2.8 IS lens was not autofocusing well with my Rebel XT
body. Often the focus would end up being way in front of what I
thought I was focusing on... I mean WAY in front, by maybe 50 ft...
although it varied.

My other lenses autofocus well and dependably with the Rebel, but not
the 17-55mm f2.8 IS.

I sent the lens in to Canon for them to fix the autofocus under
warranty. But they returned the lens saying that it was working fine.

So I tried the lens out on a friend's 20D, and sure enough, the lens
works fine with the 20D... but not with my Rebel XT. The lens is still
in its warranty period, but the Rebel is not. Not sure what to do now.
Does anyone else have this problem with the 17-55mm f2.8 IS lens on
the Rebel? I'm wondering if it is a design fault or a sample issue.

KS