For those with the a new Sigma 50mm f1.4 and 5Dmk2.
It seems that the lens front focuses at distances below 6m, but focuses perfectly at distances from 6m to infinity.
Using a lens AF adjustment setting of 18backwards, you can get it to focus perfectly from nearfield to distances of up to 3m but distances beyond will then suffer from backfocusing.
I've tested 3 lenses, 2 from one shop and one from another and they share the same problems.
It seems that the AF table for the lens has flawed calibration values.
Those who have sent it in to sigma for re-calibration: does it ensure consistent focusing for both near and far objects?
Optically, it is an excellent performer, with less CA wide open than the canon 50mm f1.4, but only if the focus is accurate. The Canon 50mm f1.4 doesn't have the focus shift issues.