From Sigma:
SIGMA 120-300mm F2.8 DG OS HSM & SIGMA USB DOCK
2013.04.12 - SIGMA 120-300mm F2.8 DG OS HSM for Canon and SIGMA USB DOCK for Canon are scheduled to start the shipment from the end of this month.
B&H has the Sigma 120-300mm f/2.8 DG OS HSM Lens for Canon available for preorder.