I recently upgraded one of my ESXi hosts from version 5.0 to 5.1. The upgrade of ESXi itself went fine (simply boot from the ESXi 5.1 install CD and select the existing ESXi 5.0 installation as the target and it gives you the option to perform an upgrade rather than a fresh install), but trying to access this subsequently with my existing vSphere 5.0 client install resulted in a message informing me certain components had to be downloaded. When I looked at what it was wanting to download it turned out to be the vSphere 5.1 client installer which I could sort of understand, so I proceeded with the upgrade of the vSphere client. Once the vSphere 5.1 client had installed I was then able to access the ESXi 5.1 host.
However, when I then tried to access one of my other ESXi 5.0 hosts the vSphere 5.1 client gave me the same message as previously and asking if I wanted to download new components. Looking at what it was downloading this time it turned out to be the vSphere 5.0 client installer! This suggests – and I couldn’t find it documented anywhere online – that the vSphere 5.1 client is not backwardly compatible with earlier ESXi hosts, which seems a bit dumb to me.
So, if you are upgrading an ESXi host from 5.0 to 5.1 and want to be able to able to manage all your ESXi hosts with the same vSphere 5.1 client, you will need to upgrade them all to ESXi 5.1.