Hi Ivan,
First take a look at this URL please:
http://www.cisco.com/en/US/partner/products/sw/secursw/ps2120/products_configuration_guide_chapter09186a00800eb0b3.html#1008066
This URL contains a document about how the ASA (Adaptive Security Algorhitm) handles traffic within the PIX, and it states that ICMP is not handled by the ASA, where IP is. So, there's a big difference in IP and ICMP handling on the PIX.
Are you using ping just to test a new setup?
- if yes, don't do that again :-)
- if no, why you need ICMP?
In case you really need ICMP returning traffic from the dmz interface to the inside interface you have to configure this properly within your PIX.
Hope this helps,
Kind Regards,
Leo