I have a pair of Pix 525 firewalls in Active/Standby. They are frontended by a different pair of firewalls that are also in A/S failover with a pair of 2960 switches between them.
When the other firewalls fail over, the new active sends out a gratuitous arp to take over the shared IP address, but it appears that the Pix didn't honor it. I haven't performed a debug on it yet since it will require some downtime. I am planning on performing some in the near future but I was hoping that somebody has seen this before.
I have done a lot of googling and Cisco searches but I haven't found much.