I thought that maybe I could contribute something to this question as a semi-professional photographer. When purchasing a camera, there are so many factors to consider. Before all else, you need to determine whether you want a point-and-shoot or a digital single lens reflex. To be honest, a high end point and shoot is capable of 99% of what a low-end digital SLR is capable of, and at less than half the size, it presents some real advantages. Point and shoot cameras to consider would be along the lines of the Canon G11.
Once you move into digital SLRs, there are a lot of small things that you will not be told by the sales person. I am a Nikon shooter, so I can really only speak to that brand, although I am positive Canon is not much different. As you move from a low-end DSLR to a high-end DSLR, you will keep the majority of features, but you'll really see a difference in a few key areas. Firstly, build quality. The cheap DSLRs are made from plastic, whereas the higher-end DSLRs have a magnesium alloy body. Next, the autofocus system. Cheap DSLRs have a slower system with fewer tracking points, and as you move up the ladder, the systems become faster and more complex. ISO performance also improves as you go up the ladder, with the best performance being seen at the D7000 for a crop (DX) sensor, and the D3s for a full-frame (FX) sensor. A full frame sensor will always outperform a crop sensor in every way, although your zoom lenses will zoom a little further on a crop sensor, usually around 1.5x the stated focal distance on any FX compatible lens.
Any Nikon DSLR below the D7000 lacks an internal focusing motor. This will make some of the nicest prime lenses Nikon makes entirely manual focus and almost impossible to use without a focusing screen - which, to my knowledge, no DSLR has from the factory.
All of that being said, the best spot to drop your money is in your lenses. Specifically the Nikon Trinity: 14-24 f/2.8, 24-70 f/2.8, and the 70-200 f/2.8 - This set will set you back almost $7,000 but is worth every penny if you want to get serious about photography. They will always be worth close to what you paid for them, and hardly depreciate. A camera body, on the other hand, is practically worthless within three years.
This barely encompasses everything there is to know, but I am happy to answer any additional questions, just drop a reply.