I found this example in the VMworld Europe presentation (excellent by the way) that would allow you to move a Virtual Machine from one virtual switch to another.

get-vm vm1 | set-networkadapter "Production"

Now say you wanted to move all vm's from the "VM Network" to "Production"

get-vm | where { ($_ | get-networkadapter).NetworkName -eq"VM Network} | set-networkadapter -networkname "Production"

This would move all VM's from the VM Network to Production.