Which is more important, the DSLR body or the lens?

I'm a beginner, and am interested in buying the Nikon d5100 or the d90. The D90 seems like it produces better quality images, however the d5100 is much more to my budget. So does it really matter if I buy a more expensive camera? Or does the key to great images lie with what lenses you purchase?

It's the ten inches behind the viewfinder.

That's like the chicken or the egg. Which is more important? The user's skill. As a beginner buy a cheaper camera. Learn as you go and acquire the lenses to suit your needs. When you're ready to upgrade the camera, stick to the brand/family so you can still use those lenses. Pretty simple.
Doesn't do you much good to keep waiting to buy something you can't afford now does it?

The first answer is right (if not helpful).It's a balance, but most photographers will tell you that the lens is more important than the body. There's a video on YouTube where they put a cheap lens on a very expensive camera and a very expensive lens on a cheap camera. The second combination gave better results.

Of course a good photographer can get a good image with an average lens and a poor photographer will waste the potential of a high level setup. Essentially what the first answer said.

Between the camera and the lens? The lens wins 90% of the time.

You have to remember that 80% of what makes a good image is outside of the camera/lens combination, any camera is just an image recording device, it records what you point it at.

But image quality is much more dependent on the lighting (the single most important thing), having an interesting subject, the composition, the 'moment' etc.etc. It's a long list. Get the 80% right and a 'point and shoot' can record a good image.

Of the 20% that's left 15% is lens and only 5% camera. With cameras the sensor is the most important part, the more up to date it is the lower the noise will be, the more sensitive the camera will be to light, in other words the better the camera is in low light, always the acid test for any camera. This currently pretty much rules out Canon cameras. Though how much longer Canon allow this situation to continue is any bodies guess.

The first 12 images in my Flickr photo-stream are taken with a Pentax Q, a camera with a tiny, but up to date sensor, this quality would have been impossible with such a small sensor just a few years ago.

There's a big difference between a D5100 and a D90 in terms of real world handling. You may be paying a little extra now, but in a few years you'll be saving money. You'll find that a D90 won't need any upgrading in the next two years, but the D5100 is crippled so badly that you'll want those missing functions and upgrade within 18 months.

It's nothing to do with image quality, but more to do with your ability to create images, and the D90 tends to free you more than a D5100.

The idea that the body doesn't matter is came about from the days of film and is a bit outdated nowadays as the sensor defiantly effects the end image. But to start with on a limited budget I believe glass still rules supreme.
To start with though id recommend investing in a really good lens, probably a 50mm prime, and a couple of other alright lenses (a wide and a telephoto) not sure what kind of photography you do but generally a 50mm will be what you fall back to for a lot of stuff. The other two will give you a bit more versatility.
As for the body any nikon or canon slr should be fine though i personally would go for something from there pro ranges. The controls tend to be far easier to use when you want to change some settings in a hurry. Though it might mean you have to buy second hand and not have quite the same low light performance as a modern camera.