Yes, I have the same doubts about the behavior during an upgrade to N5k/N9k and FEXes. I guess the answer may depend on a few variables.
In nx-os 7.2 on the N5k a VPC shutdown feature was introduced. I suppose the switch you execute this on would become the secondary and shutdown it's VPC members? If you combine this with the "vpc orphan-port suspend" on the NIC teaming ports, my guess is that these will shutdown as well. That would be quite a convenient failover strategy - just shutdown VPC then run the software update.
It would be helpful to know what status the HIFs will be in while the "install all" command reloads the N5k.
Also to be noted that there have been one or two bugs involving the orphan port (e.g. CSCvq79234), which suggests the suspend is applied during switch reload and holds until the VPC delay restore timer (150s) is finished.
Another weapon in the arsenal is "system interface shutdown", in the newer releases which closes down everything except mgmt0.
FEXes could be plugged into two upstream 5ks, but I'm more interested to know about the Straight-Through upgrade behaviour.
