VMware ESX 4.0 Update 1(Build 208167) was released on 19 Nov 2009 and I did a patch update on 3 of my vSphere Dev Servers included of 1 ESX (build 164009) server and 2 ESXi (build 164009 & 171294) servers on Dell PE2950 today.

The Redhat Linux and Windows 2003 & 2008 Virtual Machines VMware tools upgraded successful and everything look goods after the Update 1 patched.

I would suggest you refer to VMware CommunitiesvSphere Upgrade and Install forum & “vSphere 4u1 – any issues” thread before apply any VMware vSphere ESX/ESXi Update 1 patches on production environment.

Read more details about VMware ESX 4.0 Update 1 Release Notes here.

Updated:
Please make sure you check VMware Knowledge Base – Upgrading ESX 4.0 to 4.0 U1 using Update Manager fails or times out and rebooting the host results in a purple diagnostic screen
A quick note: The problem with U1 is now known to be limited to HP Proliant systems and HP Insight Management agents are running.