A new and terrific feature in Bridge CS4 is Collections. If you haven't heard of them, they are virtual folders that you can use to view your pictures in various groupings without compromising the integrity of your organizational structure. So you can pull a picture from here and from there, put it in a Collection, and use that for display or publishing. When you're done, you can discard the Collection, and there's no impact on your organizational folders.
I've published a new screencast on CS4 Collections that shows you the ins and outs. They are quite handy.
Learn more about it in The Photoshop CS4 Companion for Photographers