I started out with a free copy of Adobe Elements. Then I found a Wacom offer--buy an Intuos tablet and get Photoshop CS4 for $300. I bit.
Most folks who seem to know tell me I don't need it, that Elements will work just fine. I don't know enough to have an opinion, but that's okay because I'm slowly learning. Between the web and good books, I'll get up to speed. I do find that most instructional material is based on Photoshop and there are subtle differences with Elements--menu item location, features, and limited feature sets.
My favorite instructional material is Julieanne Kost's videos. They're packed with detail and move at just the right clip for me. Besides, she's awfully cute. She has a DVD set for CS3 and I'm waiting/hoping for a CS4 version. In the meantime, I'll build my library.
My first CS4 book is Photoshop CS4 for Nature Photographers by Ellen Anon and Josh Anon. It comes with a DVD--video tutorials and images to follow along with the exercises. I gave it a quick read and now it's time to go back and do the work. There are introductory chapters on Bridge and Adobe Camera Raw (ACR) which then lead into tools and workflows. Exposure and color adjustments, composites, creative effects, and printing round out the chapters.
With the focus on nature, we won't find any tips on portrait refinishing. All the example photos are landscape, floral, or bird shots.
I'll probably have more thoughts once I get in and start the exercises.