Hi,
You are not able to PING/ICMP the "outside" interface of the PIX from a host behind the "inside" interface. This is true for any Cisco firewall to my understanding.
The host sending ICMP Echo to an PIX interface IP address has to always be behind that interface. Basically hosts behind "inside" can PING the "inside" inteface IP address and hosts behind "outside" can PING the "outside" interface.
- Jouni