This is really a pretty easy process. To make the long story short, find your photo in the Library Module and then open it in the Develop module. It's not absolutely necessary, but you'll probably want to make the basic adjustments that it takes to make the photo look as good as possible. Then the real fun begins.

Go down to the Hue/Saturation/Luminance panel and play around with the colors in the image until you've made one large area of luminance look just the way you'd like. For instance, if it's a landscape picture, you might darken and color the sky. If the results aren't dramatic enough, try split-toning, too. When you get something you like, press the Right Mouse Button and choose Edit in Photoshop (whatever version you've set up here). As soon as the file opens, choose File > Save As and change the Edit portion of the filename to make it more descriptive of what you've done and then click OK. Your Photoshop change will become a .PSD file in the same stack as the original. So now go back to the original and use the same panels in Lightroom to make an entirely different interpretation of part of the image. Do yet another Save As and change the Edit portion of the file name to describe this version of the edit.

Now we're going to combine the effects in the two images. Open both images in Photoshop. Copy one of the images and Paste it into the other image. Now use any and all selection methods to select the area through which you wish to reveal the image below. I usually find the Select Color command to be the easiest to use for this purpose, but what you actually use will depend on what you actually want the end result to look like. Once you've made the selection, you'll probably want to feather it just enough to blend the two images smoothly. If you're using Photoshop CS3, save that step for Refine Edges so that you can blend the two photographs interactively. However you make and blend your selection, once that's done, hit the Delete/Backspace key. You can also use the Eraser, perhaps with lowered opacity, to reveal isolated parts of the underlying image.

Now you'll see the result you're looking for. If you want to take things even further, you might want to experiment with some Blend modes while the top layer is selected. If you like a Blend Mode's effect, but it seems overdone, lower the layer's Opacity.