Had a situation where I needed to reset every VM in one environment to Unlimited that had previously had a an explicitly set memory limit. Here is a nice VMware PowerCLI script that will reconfigure all the affected VMs one by one automatically for you:
get-vm|get-vmresourceconfiguration|where-object{$_.memlimitmb -ne '-1'}|set-vmresourceconfiguration -memlimitmb $null