I don't think you will need to do any messing with virtual MAC addresses. Let's say you're running off the primary right now. When the PIX fails over from primary to secondary, the failover unit takes over both the IP and the MAC address (the primary then takes over the secondary IP and MAC address). The bootom line is, whichever firewall is running, it will always be the same IP and MAC address according to the rest of your networking equipment.
Also, as I understand it, the PIX does not detect failover based on an ARP timeout. Instead, it constantly sends hello packets across all interfaces from one unit to the other.
We have a pair of 535s that failed over once or twice, and have never run into any time issues during a failover event. In fact, it was so transparent that we wouldn't have known it if our syslog server didn't send us the alerts.
Hope this helps.