SU Authentication failure

[John]

Hi

Having just upgraded to Hardy Heron 8.04, I've been trying to use a 
terminal window to su to change a configuration in /etc/bluetooth.

I keep getting an Authentication failure which I don't remember from 
Version 7.

If I change a setting in, say, the network configuration using the  
System -> Administration -> Network menu option, is asks for 
authorisation and accepts my password.

Why does su not let me in?

TIA

John