I just ran across this series of killer Photoshop tutorials by Cameron Moll, detailing how to achieve “that wicked worn look” and make your images look worn, scratched, and slightly beat-up. Like any Mac fan, I love the Aqua style, but there’s something about the worn look that just gets you. I can’t quite put my finger on it, but when it’s done right, it looks great (see his blog, linked above). The last entry in the series links to a thoughtful entry at Airbag Industries on that style.
I also wholeheartedly agree with Cameron’s recommendation of the Machine Wash Photoshop filters. They allow you to instantly apply an aged look to a layer using the included Actions. If you’re looking to create that style and don’t want to spend the time doing all the work manually, definitely check out Machine Wash filters.