I am really starting to love Lightroom 3. One of the things I just learned to do is if you are in the develop screen you can see the before and after of your work in a couple of ways.
The first way I learned was to hit the backlash “\” key. The way I remember this key combo is “I want to go BACK to the original picture”.
The other way is if you hit the “Y” key. This will give you a before and after on the same screen! This is great when you don’t want to keep having to hit “\” to see the before and after of the picture in Lightroom.