Last week a customer of mine was told to upgrade to UCS 2.2(1d)a in their environment which runs VMware vSphere 5.1. So the customer diligently upgraded the UCS Fabric Interconnects and immediately saw a problem with their vCenter VM as well as some of the other VMs. Apparently there is a bug that causes some of the fabric adapters to fail if you don't upgrade all of the blade servers to 2.2(1d)a as well and it will cause storage connectivity problems for your VMware environment. This is a known bug within the Cisco documentation.
Long story short, make sure if you do plan to upgrade that you upgrade all of the components within your UCS environment. You can do this using policies within UCS Manager to make sure everything is configured the same way. Also, make sure to check the release notes for UCS upgrades...especially those pertaining to VMware!