As far as i recall, there is always a failover cable in the box when you recieve it, from 515 models and upwards.
The failover cable is used to do a "redundant" PIX setup, where there is one active pix, and one standby pix, and in case of interface/pix failure the standby unit will take over MAC/IP and config. Today this can be done by dedicating an interface for it in the pix, so you can use the 100 meter limit on the lan cable instead of 15 on a failover serial cable.
Also the TCP/UDP sessions are transferred to the other pix, so in most cases you won't even be disconnected, unless you have some LAN/ARP problems with the failover.
Jan
Network Systems Engineer
CCNA/CQS