Hi,
It should be on by default in all interfaces
Check the output of the command "show run sysopt" and see if the Proxy ARP has been disabled on any interface.
If its disabled on some interface it should read "sysopt noproxyarp "
Also depending on your software level there is chance to change the Proxy ARP settings for a NAT configuration.
Here is the 8.4 ASA software command reference section for the command controlling interface Proxy ARP
http://www.cisco.com/en/US/docs/security/asa/asa84/command/reference/s8.html#wp1572088
- Jouni
