Vibrance is a new addition to the Lightroom Beta and is inspired from a similar control with the same name in Pixmantec Rawshooter which Adobe bought last year. I used RawShooter before I decided to switch to Photoshop CS2 Camera Raw / IView after Lightroom beta became available for Windows last summer. I have experimented with different Saturation/Vibrance combination - lowering saturation down quite a bit and raising the vibrance level to compensate and that gives sometimes good and sometimes interesting results.

The short definition for these controls from Adobe is:

Vibrance: Adjusts the saturation so that clipping is minimized as colors approach full saturation. Vibrance also prevents skintones from becoming over saturated.

Saturation: Adjusts the saturation of all image colors equally from -100 (monochrome) to +100 (double the saturation).

I sometimes use the method I described before to get an almost old style look for images like in this example:

Here the effect is used to turn down the saturation and pushing up the Vibrance to give the colors a boost. I recommend playing with these two sliders to get a feel for how they work together.