VMware has released a new support article describing the steps needed when updating from ESX 4.0 to ESX 4.0 Update 1.  If you are in the process of upgrading, you may find these helpful.

• vihostupdate Command-Line Utility—The vihostupdate command applies software updates to ESX 4.0/ESXi 4.0 hosts and installs and updates ESX/ESXi extensions such as VMkernel modules, drivers, and CIM providers. For more information, see the vSphere Upgrade Guide Update 1.
  
  Note: You cannot use the vSphere Host Update Utility to upgrade ESX 4.0 hosts. This utility is for standalone ESX 3.x hosts. A standalone host is an ESX/ESXi host that is not managed by vCenter Server. For more information, see the vSphere Upgrade Guide Update 1.
   
• esxupdate Command-Line Utility—The esxupdate command applies software updates to ESX 4.0 hosts. For more information, see About the esxupdate Utility in the ESX 4 Patch Management Guide.
  
  Download the Update Package from ESX Server 4.0.0 to ESX Server 4.0.0 Update 1 and unzip the file to an ESX host directory that has sufficient space.  Make sure you have enough room to store the update package, as well as the amount of space required.  You can verify the amount of space needed by looking at the update package with a third party archive utility.
  
  To install, run the command:
  
  esxupdate --bundle=ESX4.0.0-update01a.zip update
  
  Alternatively, unzip the file in an ESX host directory that has sufficient space. When it is extracted, there is a file metadata.zip within the offline bundle. Run the following command:
  
  esxupdate -m metadata.zip update
  
  
• VMware vCenter Update Manager—For ESX/ESXi hosts that are managed by vCenter Server. See the VMware vCenter Update Manager Administration Guide Update 1 for more information.

This article does not include steps for upgrading from ESX 3.5 and previous versions. If you are upgrading from ESX 3.x, see Upgrading to ESX 4.0 and vCenter 4.0 best practices (1009039).

More information can be found here at the source.