I would have to say that at the present time Photoshop is the better program but it would be pretty shameful for Adobe if it was inferior to Gimp. Adobe are a huge company that has millions to spend on development of their products as opposed to Gimp who somehow produce the funds to make a more than decent app and make it open source too.
I think personally the only real problem with the Gimp is the lack of a full screen environment, instead prefering floating windows which can easily get lost behind other open apps. (Hopefully Spaces for leopard can help to sort this).
I was going to also say that you can’t drag images from the finder into Gimp like you can in Photoshop. However, that isn’t the case as I (finally) figured out you can drag items from iPhoto, the finder etc onto the Gimp dock icon to open them. The only thing I don’t think you can do is drag in photos from the net to the app without saving them to the disk first. This is a small inconvenience but not a huge problem.
So yeah Photoshop is good, but The Gimp is underrated by many I think.