Discussion on:

Five ways to upgrade to ESXi 5.1

I have had the best sucess with using option #2 Interactive ISO Install/Upgrade. I recently upgraded to ESXi 5.1 HP/Dell servers.
Here's the steps:
Download the HP VMware ESXi 5.1 .ISO and the Dell VMware ESXi 5.1 .ISO, these include the drivers you need otherwise you may have slipstream them in the standard VMware .ISO.
Upgrade vCenter to 5.1 (5.1 uses multiple DBs and it's a bit more work than 5.0)
Upgrade the Hosts: Enter Maint Mode, Shutdown, (I updated Firmware), Boot .ISO, Upgrade
Upgrade VMTools on VM's
Upgrade Virtual HW on VM's (need to be powered off)
Upgrade Datastores (ESXi 5.0 and 5.1 use the same VMFS version, so this is not needed for that upgrade)

need to remove the %20 at the end of the address

we looked at the upgrade path but decided to do the clean install as those of us here had not installed the original.