Is the vsphere client 5.1 backwards compatible with earlier versions of esxi
No, each version will have its own corresponding version of the client. If you manage multiple versions, you can have quite a few clients installed.
God that is really annoying.
Oh well not too worry
Yes and no. You would need to installation for each corresponding version but you can have them all installed on one computer. I publish the vSphere Console via Citrix and have it working for 5 and 5.1 without incident.
1. You can just run the 5.1 installer over the top of an existing 5 installation and it will work for both versions.
2. You can install 5.1 and then try to access a 5 farm and the bits will be added to the existing version.
Technically, what the gentleman said above is accurate but I believe you were asking whether a single system with the vSphere client could access multiple versions which is indeed the case.
I also find this extremely annoying especially when I have heaps of clients all using different ESX versions.
Is this a technical limitation or by design?