In the old days, you would use the Magic Wand tool to make the selection. Then simply Click on the Add Layer Mask icon at the bottom of the layers panel.
Is there a new way? You bet.
CS4 has a bunch of new features, but this is one of my favourites. I do a lot of composition work and making masks quickly has always been a challenge. Until now, that is ...

Now, just make the selection and go to the Masks panel. Then select the Pixel Mask icon. Photoshop CS4 does all the work for you. You've even got a ton of controls for refining the mask that were never there before. How cool is that!