normally, anything leaving a pix can get back in, but for Ping/Trace responses add the following -adapt for your setup.
access-list OUTSIDE_INBOUND_ACL line 1 remark Permit ICMP Responses
access-list OUTSIDE_INBOUND_ACL line 2 permit icmp any interface outside unreachable
access-list OUTSIDE_INBOUND_ACL line 3 permit icmp any interface outside time-exceeded
access-list OUTSIDE_INBOUND_ACL line 4 permit icmp any interface outside echo-reply
don't forget to apply the acl to the outside interface if you don't already have an outside acl.
