From Sigma Japan (paraphrased from Google Translate):
SIGMA 120-300mm F2.8 DG OS HSM, SIGMA USB DOCK Release Postponed
Thank you for your patronage.
The "SIGMA USB DOCK" and "SIGMA 120-300mm F2.8 DG OS HSM" lens were expected to be released in March 2013 for Canon. Because of a delay in the procurement of [manufacturing] parts, the release date has been postponed.
For now, the revised release date has not been determined. Please check our website for future updates on these products.
We apologize for the inconvenience.