The gun establishments I go to generally have separate rules for carry guns or guns you're bringing in. One of the guys at the local-ish range said they basically don't care if it's cased, CCW, or open carry, but it better be in a safe position (basically, trigger guard covered and it's not pointing at anyone).
I personally am of the belief that, regardless of whether the sign is posted "to stop bad guys" (like most of us seem to think people post that sign for) or like Double said, that it's posted to prevent NDs (someone stupid enough to cause an ND probably didn't read the 4 rules, though, so they probably wouldn't read the sign), I would personally rather not be disarmed.
Mgkdrgn, I would personally have said "recommended" instead of "permited". But I love that sign.