According to the 7.0 command reference, Cisco no longer recommends using the alias command:
"To manually translate an address and perform DNS reply modification, use the alias command in global configuration mode. To remove an alias command, use the no form of this command. This command functionality has been replaced by outside NAT commands, including the nat and static commands with the dns keyword. We recommend that you use outside NAT instead of the alias command."
http://www.cisco.com/en/US/docs/security/asa/asa70/command/reference/ab.html#wp1343428