Generally, no. There's not a magic answer key out there. It gets a little easier once you realize they have a few lines of cameras and can distinguish between the lines - but these get a bit blurred when they change something enough that they introduce a new line. A lot of the models you see are successors to previous generations - if you limit your searches to only current models, its a bit saner.

Part of it is because one camera may be better at one thing and worse at another. The Canon 7D is supposed to have great autofocus tracking but poorer image quality than their 5D mkII - which is more important depends on what you want. On the D7000 vs the D5100 - the D7000 is certainly a better camera, but the image quality between the two is probably pretty minimal. You're paying for better usability and better build, but the model number itself doesn't really indicate whether that's an important criteria to you.

One of the main things in my answer above is that just because a camera may appear technically better in many areas, it may be a tradeoff in others and that may be the important piece to you. You can't judge pretty much anything strictly based on model numbers.