1) Download "upgrade-from-esxi4.0-to-4.1-update01-*.zip" from vmware.com

2) Download & Install "VmWare CLI" from vmware.com

3) Check for the Bulletin-Names

C:\Program Files (x86)\VMware\VMware vSphere CLI\bin>
vihostupdate.pl --server <IP_or_DNS> -l -b c:\temp\upgrade-from-esxi4.0-to-4.1-update01-348481.zip

Enter username: 
Enter password:

---------Bulletin ID---------   ----------------Summary-----------------
ESXi40-TO-ESXi41UPDATE01        Upgrade from ESXi 4.0 to ESXi 4.1 Update 01
ESXi410-GA-esxupdate            ESXi pre-upgrade Bulletin

4) Apply the PreUpgrade-Package

C:\Program Files (x86)\VMware\VMware vSphere CLI\bin>
vihostupdate.pl --server <IP_or_DNS> -i -b c:\temp\upgrade-from-esxi4.0-to-4.1-update01-348481.zip -B ESXi410-GA-esxupdate

Enter username: 
Enter password:

Host updated successfully.

5) Apply the Firmware-Upgrade

C:\Program Files (x86)\VMware\VMware vSphere CLI\bin>
vihostupdate.pl --server <IP_or_DNS> -i -b c:\temp\upgrade-from-esxi4.0-to-4.1-update01-348481.zip -B ESXi40-TO-ESXi41UPDATE01

Enter username: 
Enter password:

The update completed successfully, but the system needs to be rebooted for the changes to be effective.

6) Reboot the ESXi-Host