Dates are obviously subject to change, but the last I heard from Cisco is that this would be targeted for Q3 CY13. As with the front-to-back airflow on the Nexus 5596UP, there will be a requirement for different power supplies and fan modules as well as the NX-OS support.
Finally Cisco added support for front-back airflow for the 5596T on the NX-OS Release 6.02
Cisco Nexus 1100 W AC front-to-back power supply (PID: NXA-PAC-1100W)
Cisco Nexus 5596UP/5596T Fan Module, front-to-back Airflow (PID: N5596UP-FAN)