In the episode where BeBe and Frasier do a talk show with the monkey ( Shown in May ), at the very begin in the Coffee Shop, Frasier is wearing his wedding band ( the credits are still showing). You have to look closely.
No, I agree that its a slip-up. People do not usually go around wearing their wedding rings (especially men!) if they are divorced. That's ridiculous. I'm sure its his real wedding band that he forgot to take off...or wore for a sentimental reason, like an anniversary or something.