Hi Anthony,
It looks like SSO is not supported when doing Fast Software Upgrades. This is because when you upgrade both MSFCs then reboot the standby, the two IOS versions become different and fall back to RPR.
Here is an excerpt from CCO:
"The fast software upgrade allows you to reduce planned downtime for software upgrades or downgrades. The fast software upgrade procedure consists of loading a new image onto both the standby MSFC and the active MSFC, and then rebooting the standby MSFC. The new image running on the standby MSFC is incompatible with the image currently running on the active MSFC, so the standby MSFC comes up in RPR mode. The fast software upgrade is done in RPR mode to avoid image incompatibility during the upgrade process.
To bring the new images into service, the standby MSFC is forced to switch over by taking the active MSFC out of service; the standby MSFC now becomes the active MSFC. Next, the out-of-service MSFC is allowed to boot; it becomes the standby MSFC but runs the newly upgraded image. The new image is running on both MSFCs, and the standby MSFC comes up in the hot-standby state. At this point, the system is now running in SSO mode because both MSFCs are running the same image version."
http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a0080462647.html#wp1064945
Hope this clears things up.
Michael.
