I too love free software, but finally I had to bit the bullet and get Photoshop. Gimp is great, but not for professional image processing. The most glaring things left out:

• Supports only 8 bits. My scanner has 16 bits/channel. I have been using cinepaint, which now is going through a transition period, but Photoshop is definitely nicer than cinepaint for photo editing.
• No support for color profiles. This is a killer if you want to do any kind of digital darkroom with some accuracy.
• No decent support for stitching photos to make panoramics. Before you say that you can twiddle with layers to do this, go see how Photoshop handles this, there is a huge difference. Photoshop can detect similar areas and distorts the photos (to make up for perspective change and lens distortion) to stitch them together properly. In GIMP it's hopeless.

Aside from this, GIMP has more than its share of bugs. Just yesterday I was doing a complicated selection from an image, and trying to bucket-fill it with solid color. For unknown reasons the filling would alter also non-selected areas. Go figure. In Photoshop this worked fine.

I use linux for everything else, but for photo editing, Photoshop IS much better. Also, the GIMP code is an undocumented mess. At some point in time, I wanted to hack into it to add some functionality, and I spent 2-3 hours staring at the code without being able to figure out how to access the image pixels. At that point, open or closed source, what's the difference to me?