You need to permit ICMP in the outside access-list.
See: Handling ICMP Pings with the PIX Firewall
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a0080094e8a.shtml
The PIX and the traceroute Command
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a00800e9312.shtml
example:
access-list 101 permit icmp any host YourPublicIP unreachable
access-list 101 permit icmp any host YourPublicIP time-exceeded
access-list 101 permit icmp any host YourPublicIP echo-reply
Note replace the <101> with your ACL name.
sincerely
Patrick