On the ESXi hosts
1) Open a shell to each host
2) Delete /etc/vmware/vmware.lic & license.cfg
3) restart services
For vCenter
1) Create a DSN to your local SQL Express instance that holds your vCenter DB.
2) Uninstall virtual center
3) Re-install virtual center and point to your DSN making sure not to overwrite.
With this method, I have been able to refresh my 4.1 and 5.0 hosts. Have not confirmed if this works for 5.1 first.
Sources
http://shanseworld.blogspot.com/2009/12/vcenter-reinstallation-after-60-days.html
http://yuridejager.wordpress.com/2011/07/18/reset-vmware-esxi-4-1-trial-license-period/
