I have a late 2006 white 13" macbook with a 2.0 C2D, 2GB DDR2 ram, 80GB 5400rpm and a GMA950. It's been thoroughly put through its paces the past 3.5 years and has gotten me through the entirety of my undergraduate career.
In that time I've used Adobe CS3 extensively, both photoshop and illustrator. Even on battery power and with very, very large files open in PS I was able to multitask with safari, pages, itunes, and photoshop and have 1.5-2 hrs of battery life. Oh, and btw, the performance of photoshop in this situation was still quite acceptable; I was able to apply large filters to 40-50 layer, 8.5x11 images within ~20 seconds.
And this is on a computer that is significantly less capable than the 13" MBP available now.
The combination of improved processor architecture/speed, DDR3 ram, larger hard drives, VASTLY superior graphics chips, and the sexy, LED backlit screen/aluminum body puts the 13MBP leagues ahead of what I have and will definitely be capable of running CS4.
Needless to say, I'll be getting a new, base model 13" MBP imminently.