In answer to the original question: I'd keep my distance from someone in a committed, mono relationship.
I came out of lurking to comment on the flirting conversation, though. I'm floored. I've never really considered the "I'm wearing a ring, therefore I'm only joking." angle. I've always worried that women would think "He's wearing a ring, therefore he's a (wannabe) cheating #&^%#$." I always try really hard not to do it. Unfortunately, I am very flirtatious by nature.