One of the more frustrating aspects of Lightroom 1.x for me has been the inability to use vignetting on cropped images. Lightroom 2.0 gives us two ways to handle this - a Post-Crop control, and Retouching brush.
The Post-Crop vignette works exactly like the previous vignetting control, but only on the cropped area of your image, making it actually useful now.

It’s pretty simple, and if you’ve played with the original tool at all, you’ll be right at home here - negative numbers darken, positive lightens. The midpoint controls how far from the edge the effect starts to take place.
The Retouch brush gives you more control though, letting you paint your adjustment right on the image similar to the dodge and burn tool in Photoshop. For this you’ll want to be sure and turn off Auto Mask to ensure the adjustment is applied to the entire area.

It takes a bit of experimentation to get the right levels, but I usually end up between -.5 and -.8 Exposure adjustment with a Feather of about 60-80, and a Flow of 50-75. This gives me nice soft transitions in the vignette for a more natural feel.
Using the Retouch brush works especially well when you don’t want to apply the effect as strongly in all four corners of the image. In the sample here, I’ve gone lighter in the bottom corners and a bit heavier in the sky to keep the eye contained without losing details in the foreground.