You know the old saying "if it's too good to be true, then it probably is"? That's very important to remember when shopping online for a digital camera.
Let's say you're browsing the price listings for a PowerShot G3. While most everyone else is selling them for $799,
you see one place selling it for $599. You order them and here's what probably happens next:
They call you and try to hard sell you accessories like filters, batteries, and carrying cases
They try to charge you extra for things that should have been in the box, like the software and cables
They will tell you that the camera you've bought is "gray market", meaning not sold in the US, and not covered under the manufacturer's warranty
At this point, if you refuse to buy it, they will either hang up on you, or tell you that the camera is out of stock.
This may sound funny, but 9 times out of 10, these resellers will be located in New York City. They lure you in with a low price and then charge you $100 for shipping, or $200 more for the US model. My advice: read ratings of the merchants on PriceGrabber and Reseller Ratings, before you get burned!