I have the same problem with the Quantaray version of this lens, Sigma manufactures.  I did testing and compared it to my Nikkor lens and it could not produce a focus shot at the same settings as the Nikkor.  I have to have the camera set manually at f/11 or higher and 1/200 or faster with a flash indoors to get a focused shot and if the subject is moving at all even faster.  I wish I had not wasted my $359.00.  I thought about sending it in to the manufacturer. It came with a 5 year full warranty but they will probably send it back and blame it on me or the settings.  The auto feature will not correct for it either.  I never had a problem with my film Sigma lens.  They have not gotten their digital stuff right at all.  I will never buy Sigma or Quantaray again. 

That's right.
It's a cheap lens.
Buy the 16-35/2.8 and 70-200/2.8: lot better

atdlee@netzero.com