I don't believe that a spanning tree recalculation won't occur due to the same mac-address being learned on a different port. The STP recalc is done only as ports become active and inactive, as the pixes failover the ports do not do that unless the pix code rebooting on one unit causes the fialover.
The switch ports are always active as long as the pix code is active (both the active and standby units) The pix failover hearbeat signal between them runs over the enet interfaces as well as the serial cable.
The pix documentaion recommends that the portfast feature be enabled on cisco cat switches when configuring ports for pix interfaces to allow the port to go into forward mode as soon as the pix activates its interface. This is to allow for the situaion to achieve fast pix failover status convergence where a pix reboots - and some failovers will cause the pix to reboot.
It is my understanding that the new rapid stp protocol is now standardized and the newer non-cisco switches should have a similar capability too.
I recommend that either portfast or rapid stp be configured on the switches and switchports that the pix units connect to (for all interfaces).
I hope this clears the issue up for you. If not, let me know what other help you need.
