Last jam night I brought my fretless to, one guy got up to play bass... I was trying to tell him it was fretless, but the guitarist was talking his ear off. He gets up, puts it on and looks down at it... confused. "Oh, this is a fretless!" Actually, he did a nice job on it anyways, could hardly tell that he hadn't played one before!
Yeah same identical thing happened to me. There was this duo (bass-guitar singers) had this gig with a jam afterwards. So I'm playing bass at the jam (usual Carvin AC50 fretless with lines) and the bass player from the duo comes up and wants to try my bass. Sure, I say. He looks at it and fiddles around a bit and then really did a credible job playing a fretless for the first time. Always makes me think that going fretless can't be THAT difficult if a fretted player can just pick one up and sound decent.