You do not really 2 pNIC for each node. You ony need 2 vNIC.
My question is can I put the public vnic of the 2008 clustered vms on the same port group as my other vms?
Yes, as written before.
Just use the same network for other VM.
setup a physical nic and portgroup for the private nic on each esx host with the physical nic connected to an isolated switch
how do I do this?
You do not really a isolated switch, but if you have a spare pNIC you can create a new vSwitch ad use an external switch to connect them (this is a way to implement an isolated switch across two or more ESX).
But you can simple create a new VM portgroup on the same vSwitch for other VM.
See also:
http://www.vmware.com/files/pdf/virtual_networking_concepts.pdf
Andre