HOW TO TELL CAMERA RAW WHICH PHOTO TO USE FOR EDITS
When you open multiple RAW images in Camera Raw, if you press Command-A (PC: Control-A) to select them all, any change you make to the top image in the list will be made to all the other selected images. But what if you'd prefer to edit the fourth or fifth image down, and have all the rest adjusted the same way (rather than having to adjust the first photo in the list)? It's easy-once all the photos are selected, Option-click (PC: Alt-click) on the photo along the left side that you want to base all your edits on. That photo will now appear in the preview window, and changes you make to it will also be applied to all other selected RAW photos.
From Photoshop Killer Tips