Initial vanilla install of ISA did not allow me to ICMP through my server. Both external and internal nics respond to ICMP's but will not pass through the server. I followed the instructions on ISAServer.org and created rules opening everything up, all services (telnet, ftp, etc..) work with the exception of ICMP.
Enable IP Routing
Configure the client as a SecureNAT client.
Then you'll be able to ping through the ISA Server.