Thinking about this yesterday - I believe that its actually a problem caused on the server side of things - I can find plenty of examples where someones domain with and without
www. are the same even down to the backlinks listed are exactly the same - because of the server config where the domain resides
On others where the server is not set up to redirect correctly, the backlinks are definitely different and PR shows differently
I say not set up correctly - but Im not sure yet which is the correct way