I share the same problem on my iBook but i have no answer.
I also share the same freezing phenomenon when i try to open NetWork Accessory. If i open the console, i can see never ending timeout on the process lookupd. Killing it gives access to the NetWork Accessory but it is then impossible to login for changing parameters.
I've found that the global reaction of my IBook on OS X is worst since i changed my network parameters (i try a ppp connect that doesn't work on top!).