We have recently updated KB article: Collecting diagnostic information for VMware vCenter Server (1011641) with vCenter Server 5.1 information, says Joel Yaross, Technical Support Engineer in our Broomfield Colorado office.
"If you're using the vSphere Web Client, you can select hosts from which to generate and download system log files with the option to include vCenter Server and vSphere Web client logs. For more information, see KB: Collecting diagnostic information for ESX/ESXi hosts and vCenter Server using the vSphere Web Client (2032892).
Note: You can also you use the Log Browser feature in the vSphere Web Client to view, search, and export one or more vCenter Server or ESXi log files. For more information, see Using the Log Browser to view, search, and export Logs for troubleshooting (2032888)."