It's a completely new lens, rather than a revision of Sigma's existing 50mm F1.4 EX DG HSM, with an unusually complex optical formula for its type (13 elements in 8 groups). It's also rather large, with a strong family resemblance to the excellent 35mm F1.4 DG HSM. The 50mm F1.4 will be available in Canon, Nikon, Sigma and Sony mounts.
The lens employs an ultrasonic-type focusing motor, and according to Sigma uses improved autofocus algorithms. It's also compatible with Sigma's USB dock (allowing the user to update the firmware when required), and Sigma's Mount Conversion service. Pricing and availability are still to be confirmed.