There are, of course, a few new capabilities, though they'll probably be handy for only a limited group of users. You can export images for Zoomify, which deconstructs images into smaller tiles and creates a small Flash movie that you can embed into a Web page, effectively allowing you to zoom in on larger images. And the automatic Photomerge will probably be a hit with panorama constructors.
For users new to image editing, Photoshop's rather monolithic learning curve remains, and you're better off starting with an inexpensive application such as Adobe Photoshop Elements 5 or Corel Paint Shop Pro XI. One of those may be all you need. But if you've been waiting to take the plunge, Adobe Photoshop CS3 is as good as any version to dive into, and if you're a pro considering the upgrade, it's probably worth the $300 or so bucks it will cost you. As for enthusiasts, that difficult decision remains between you and your wallet.