Accepted Solutions
1. Is there any easy way for me to find out via powercli or vSphere where each server boots from? (Preferrably centrally - don't want to have to run commands on each and every host individually)
Hmmm.. yes. These steps would help you, but it is a round about method than checking on each servers individually:
[For checking if the server is usb boot or not, just type esxcfg-info -e]
1. In the vCLI, you can call the esxcfg-info -e as the additional command passing the list of servers from an array. for example:
esxcli --server myESXi --username user1 --password 'my_password' storage nfs list
2. Can you use VUM to upgrade USB and/or SD card installations?
Yes sir, you can..
3. Any known GOTCHAS for updating between these two versions?
If you are mentioning between USB/SD card, then nope. ESXi sees all as storage units.
.......I was thinking that it's probably best to install a seperate USB with 5.0 U2 instead of using VUM so the old USB stick can be my rollback. Wise? I assume doing this means it loses its configuration?
Unless, your BIOS boot manager shows the USBs as two separate bootable entities. which I highly doubt 
4. How best to backup and restore the host configuration if I swap out the USB?
You can use vicfg-backup from the vcli utility to back up your configuration and restore it back
1. Is there any easy way for me to find out via powercli or vSphere where each server boots from? (Preferrably centrally - don't want to have to run commands on each and every host individually)
Hmmm.. yes. These steps would help you, but it is a round about method than checking on each servers individually:
[For checking if the server is usb boot or not, just type esxcfg-info -e]
1. In the vCLI, you can call the esxcfg-info -e as the additional command passing the list of servers from an array. for example:
esxcli --server myESXi --username user1 --password 'my_password' storage nfs list
2. Can you use VUM to upgrade USB and/or SD card installations?
Yes sir, you can..
3. Any known GOTCHAS for updating between these two versions?
If you are mentioning between USB/SD card, then nope. ESXi sees all as storage units.
.......I was thinking that it's probably best to install a seperate USB with 5.0 U2 instead of using VUM so the old USB stick can be my rollback. Wise? I assume doing this means it loses its configuration?
Unless, your BIOS boot manager shows the USBs as two separate bootable entities. which I highly doubt
4. How best to backup and restore the host configuration if I swap out the USB?
You can use vicfg-backup from the vcli utility to back up your configuration and restore it back