Just a quick post for those that may not have heard about this VMware Alert.
If you are running vSphere 5.5 and have NFS datastores, it is advised NOT to upgrade or patch to Update 1. NFS disconnections have been reported after upgrading and can lead to freezes and crashes on Virtual Machines located on the NFS datastores.
You can read a good write up on the issue by Michael Webster on LongWhiteClouds.com.
The official KB from VMware is here:
KB 2076392 - Frequent NFS APDs after upgrading ESXi to 5.5 U1