There are things on the customs cards that they state if you have too many of you may have to pay some kind of duty. Having said that, even if you have less than the amount they could notice this on an xray scan and investigate.
I brought back some bottles of Hooch (yeah, the alcopop drinks, some friends liked them), only about 6 of them and at US customs they had me go to the Red customs line (I think it was anyway, this was many years ago so I'm a little hazy on the details) and they opened it up. Because I was well within the limit they didn't do anything but it may still be worth declaring things like alcohol, ciggies, etc.
Of course, if you are within the limit you could probably go through the regular line and the worst they could do is ask you to open your luggage.
Matt