How to upgrade your ESXi 4.0U2 to ESXi 4.1. Contrary to what some may believe. VMware's documentation is not the simplest to navigate sometimes. However, this is what happens when you have technical writers typing up stuff for Systems Engineers! We don't need 10 pages of text to say the following!
You need vSphere CLI - Download Here vCLI
For Windows or Linux and install on your workstation.
Download the Upgrade Package for 4.0 to 4.1 or the Upgrade Package from 3.5 to 4.1
Power off your VM's and put the host into maintenance mode from the vSphere CLI prompt:
vihostupdate.pl --server hostname or IP -i -b c:\temp\upgrade-from-ESXi4.0-to-4.1.0-0.0.260247-release.zip -B ESXi410-GA
vihostupdate.pl --server hostname or IP -i -b c:\temp\upgrade-from-ESXi4.0-to-4.1.0-0.0.260247-release.zip -B ESXi410-GA-esxupdate
Thanks to Sebastian: http://blog.jargontech.com/?p=19
Thanks to MicroKid: http://communities.vmware.com/message/1570298
Thanks to Peacon: http://blog.peacon.co.uk/wiki/Upgrading_ESXi_4_to_4.1
VMware vCLI Reference: http://www.vmware.com/support/developer/vcli/vcli41/doc/reference/index.html
Just FYI for new CLI folks on Windows. Make sure you cd into the /bin directory under the vSphere CLI so that you can run the Perl commands or just create an alias in Powershell.
Post installation. You will get a pop-up Window.
The required client support files need to be retrieved from the server 'vsphereclient.vmware.com' and installed. Click Run the Installer or Save the Installer.
I would recommend just running it if you have time.It is just going to update your vClient on your Windows/Linux machine.