A new patch has been made available for ESX 4.0 Update 1, this is called Update 1A. It only affects ESX and not ESXi. Here is an except of the alert put out by VMware:

ESX 4.0, Update 1, Alert: Upgrading ESX 4.0 to 4.0 U1 can fail or time out and leave the host in an unusable state if using HP Systems Insight Management Agent. ESX 4.0 Update 1a (a re-release of ESX 4.0 Update1) that addresses this issue is available. Please read KB article (ID 1016070) before proceeding with the upgrade.

As I said above, this patch is listed as ESX 4.0 Update 1A and can be found on the VMware Downloads website, or from within VMware Update Manager.