Having a tripod would be very convenient for steady night shots on your DSLR camera. However, with all the tightened security, finding a place to use the tripod/monopod is tricky.  I prefer taking a monopod for the convenience factor (fits in a backpack) espeically when you are travelling by subway.

My current monopod setup is the Manfrotto 790B which I purchased for ~$30. Its very compact at 15″ folded. http://www.amazon.com/Manfrotto-790B-Modo-Photo-Video-Monopod/dp/B000FA7PQC

Tripods and monopods are NOT allowed at:
Empire State Building (confirmed)
Statue of Liberty (confirmed, really strict security)
Many museums
Theaters (no photography allowed)
Times Square (tripping hazard, lots of police around that area checking)

My fav. locations to take photos in NYC are the Empire State Building 86th floor (since its open-air, no glass reflections to worry about), Times Square, and the Statue of Liberty.

Possible locations to use a tripod/monopod in NYC:
Grand Central Station
Central Park
Ellis Island
Brooklyn Bridge
Rockefeller Plaza
SOHO area is another possible location too

So, what to do if you can’t use a monopod or tripod? I suggest using a fast lens (f4 or faster) that has image stabilization in it. You can also “steady” the camera by placing it on a rail/concrete wall/etc. Thats why night photography is a challenge…