As you know conduits apply to the whole PIX, not just an interface (when used with a static command statement, a conduit command statement permits users on a lower security interface to access a higher security interface. When not used with a static command statement, a conduit command statement permits both inbound and outbound access). So, you can try "conduit deny icmp x.x.x.x any echo" and "conduit permit icmp any any". That will prevent anyone from pinging your pix outside IP and will allow all other icmp (can block other icmps if required).
Access-lists are a lot easier to work with and can apply to only one interface (eg outside), so you may want to migrate them.
Hope it helps.
Steve
