for me the biggest difference is the new 3D viewing and manipulating function ( I do 3D work as well as 2D graphics). Other than that, the whole CS4 Suite is basically just an expensive patch. Most of the programs have added 3D stuff, but personally I won't be using CS4 outside of work/school. I'll wait until something changes worth the money.
(all of my stuff is legal, I am a professional designer and the price is worth the software)