Hot off the presses is a Cisco ‘announcement’ of a virtual ASA product that is in the works, although no details were released about pricing and availability. It will leverage the capabilities of the Nexus 1000v DVS, and is more for edge protection (North-South traffic), vice internal traffic (East-West) which their VSG product is better suited for. You can see the full blog post from Cisco here.
The advantage of Cisco ASA appliance is that it can be partitioned into multiple virtual firewalls known also as Security Contexts. Each of the later acts as a separate firewall with its own security policy, interfaces and configuration. But, features like IPSEC and SSL VPN, Dynamic Routing Protocols, Multicast and Threat Detection are missing.