The one caveat to the above is if you're leaving and returning to the US. If you have expensive stuff customs will sometimes ask where you bought it because they want you to pay taxes on it. On my way back from Tokyo (interestingly enough, on my way in I wasn't even stopped) it was just simple questions. Did you buy any of this in Tokyo, what's the value, etc... A friend of mines dad does a lot of travel photography and brings cases of kit and always brings all his receipts to avoid customs hassle on his way back into the country. In this case, I think it depends on just how much stuff you have, ie, a camera and a few lens shouldn't be an issue.
On my way into Jamaica I got fully shaken down and had to pay $50 in 'duty fees.' Although, I didn't just have a camera and had a bunch of other stuff that they wanted to say I was trying to sell (he specifically pointed out that I had more than 1 camera lens so obviously I was selling them /sigh). Anyways, long story short it was a shakedown and I just had to negotiate what I was going to pay. Third world countries FTW! lol