First of all, I doubt that Nokia ever patched the firmware properly, although this certainly is speculation. I just can tell you that I haven't seen a Nokia 6310/6310i that is not vulnerable to the Blugbugger attack yet - I mean it's a pretty old model anyway. What about buying a used one at ebay or so? If you just need it for demonstration purposes it does not need to be a brand new model. As of the firmware downgrading, I have to admit that I have no clue actually. Never dealed with this topic before - I'm sorry.
If I buy a new one will it still have this flaw or will the firmware patch this? If the flaw is fixed is in possible to downgrade the firmware?