I have dealt with the following online camera merchants:

B&H Photo & Video ( http://www.bhphotovideo.com ) (by far the most highly rated mail order camera shop - I love 'em)

Adorama (http://www.adorama.com ) (good reviews, bought a few things from them, never a problem)

17th Street Photo ( http://www.17photo.com ) (bought a Nikon flash based on their price and high reputation - great to work with - no problems whatsoever)

Check out http://www.reselleratings.com and http://www.epinions.com for their reviews of these guys.

Just make sure that you get a product that carries a warranty for the area in which you live. Nikon USA, for example, will not perform service on a camera that was purchased with an non-USA International Warranty. This is so they can control distribution of their products, apparently.