I have CS2 on my desktop, and CS3 on my laptop. I know how to use the programs pretty good, though i'm no expert.
Here are the main things i noticed, although there are ALOT more for the more dedicated users:
The interface on CS3 is alot easier to use and actually "simpler" looking. It also seems to run alot smoother and faster(maybe its the computer though?).
The brightness and contrast controls are changed a bit and make photos look alot better, where sometimes they could kinda mess up the image.
The print menu is alot better, and easier to use with more controls right on the print preview page.