I'm not sure how i went so long without having dual monitor support for editing photo's. This support was essentially non-existant in Aperture. Lightroom handles this in an awesome fashion.
You can run Lightroom on your primary monitor in either standard mode or fullscreen mode. Regardless of how you run it on your primary monitor, you can output the photo's to a secondary monitor for previewing. In the following photo, you can see that I am in Grid view on my primary monitor. In this view, I can rate and add my keywords. I can move between photo's on my primary monitor and view the fullsize photo on my laptop's screen.
This feature works great can be accessed from the dual monitor icon on the bottom left of the screen.
The more I use Lightroom, and get used to the different interface, the more I like it and find myself surprised at how long I put up with Apertures lack-luster feature set (relative to Lightrooms).