just tried one on my new Oly E3 and the lens had a front focus issue of about 8 inches,, would not focus on infinity and obtain anything clear,,
know others that love this lens and was wondering if anyone else has had a front or back focus problem with theirs,, I sent it back to B&H for a replacement and waiting for the next lens to arrive,,
a call to Sigma in New York indicated they knew of no problems on any camera,,
perhaps it was just one of a lot that was out of whack,,
It does happen sometimes with lenses, I guess you just got unlucky. With one this far out you've done the sensible thing in returning it for a replacement, if you have one that is just out then Sigma can recalibrate it for your camera.
I've had no problems at all with mine on a Canon.