Most clubs will not allow photography during biz hours by hobbyists. During non operational hours maybe. One way to do it is to work out a deal with the club. You are allowed to take the photos and keep them but then must give one set to them for their publicity use. They might find it agreeable.
From my understanding, big clubs like Zouk only has authorised photographers or rather last I heard one authorised photographer who is allowed to take photos during operational hours.
PNS they will normally close an eye. Cos your PNS wont be able to capture much in the darkness anyway. And unless other patrons make an complain about you flashing away and affecting their "mood", the bouncers will leave you alone. After all a club is where people go to party and celebrate and wanting to take pics is normal. But once you wipe out a DSLR or SLR and start pointing at the crowd, the decor, the DJ.... that one u will most likely get stopped.