If you are running PowerPath/VE, I noticed the follwing tweet from @VMwareKB
***ALERT: Important KB affecting customers with PowerPath/VE. See:bit.ly/OE3JMS please RT ***
There is an issue where PowerPath/VE 5.7 fails to load in a vSphere 5.1 environment.
The recommended action is to hold off with the 5.1 upgrade if you are using PowerPath/VE 5.7 and wait for EMC or VMware to release a patch fixing the issue.