Andy, unfortuantely at this time PIX physical interface can only be configured with one Ip address, if you needed the interface IP be different from what currently is you would need to re-address the interface with desire IP address.
There are other scenarios where a physical interface may be splited into logical interface such in configurations entailing dual ISP backup where you may have outside physical interface and off it logical interfaces and still in this scenarion a logical interface is configured with only one IP adddress. I hope I have answered your concern.
Rgds
Jorge
Jorge Rodriguez