After some research I think this is related to the LSI Megaraid Controller.
From the ESX3.5 U4 release notes:
Log is filled with "storelib Physical Device Device ID" messages
In some cases, providers using LSI Logic drivers (megaraid2, megaraid_sas, mptscsi) repeatedly log messages such as "... cimprovagt: storelib Physical Device Device ID" and fill up the /var/log/messages log. These messages are part of debug code and should not be logged at this location.