Are you sure its the lens ?, I have both the 120-300 sigma, and 70-200 canon, when shooting motorsport, I get the same "fuzziness", with both, on my 40d.
If I shoot a burst of 4 shots at a bike or car, most of the time one or two of them are slightly out of focus, (I have tried with both lenses), sometimes slightly in front, sometimes slightly behind.
I could understand this if the subject was coming towards me or going away, but it also happens when the bikes are on a race track, crossing at 90 degrees to me, at a constant distance.
I thought at first it was my fault, but I can not remember getting as many wrong on the 20d I had before, and there are now many users complaining on various forums about this problem with the 40d. Some have sent their cameras back to have the focusing re calibrated, some successfully, but some are reporting no improvement.
As the 50d uses a similar focusing system to the 40d, the "problem" may still be there.
I was going to send my 40d to be re calibrated, but when I finaly decided to send it, it was 4 days out of warranty, so I will probably have to pay to get it done.
This cuts down on choices for my next upgrade, in a few months, I was thinking of going for a 1dmk3, for the improved focusing, but now people are reporting problems with some of them (something to do with the magnets in the shutter assembly, no problem if it fails in the warranty period, but expensive after), the 5d mk2 also seems to have the same focusing system as the 40d/50d so it remains to be seen if that suffers the same issues.
I have been seriously considering changing to Nikon because of this, but it would cost a hell of a lot to replace my lenses, ( or even the new sony alpha 900, it also looks interesting).
Alwyn