I agree with furious... I think that the heat is the number one motivator for removing your shirt. I party it up in NYC, Philly, and DC (I live in South Jersey), and many people with good bodies and some with not so good bodies take off their shirts after an hour or so.
I usually wear tight sleeveless shirts to clubs so I can show off my hard work, but show a little class as well. However, I also dance at some clubs on the boxes and when it gets really hot I take off my shirt because it is more comfortable.
I can understand that there will always be player-haters out there that are going to look down on people removing their shirts as cocky bastards, but most people go to these clubs for the same reason - to meet people and have fun. Don't be just another hater.
Now, as far as bars/lounges are concerned (which is what I think some of you are thinking of when we say clubs) - then it IS inappropriate to remove your shirt. It is a different atmosphere. In dance clubs everyone is furiously dancing, in bars and lounges there may be some dancing, but it is mainly talking.