The P3's form factor is relatively unchanged from the P2: it's now made of aluminum (with an unseemly black bezel around the screen, unfortunately) and the dedicated volume buttons are moved to the top. The major exterior hardware upgrades are the addition of a small speaker and haptic feedback on the (capacitive) touchscreen, so it'll vibrate when you tap it. The GUI, however, is a real step up for Samsung.
The P3's home screen is almost a desktop environment, with movable, interactive widgets and icons and multiple screens a la the iPhone and T-Mobile G1. Icons can be dragged from a disappearing bar onto the desktop and back. I'm really liking the little lightbulb that changes the amount of backlighting when tapped. The P3 shares the P2's excellent 480x272 screen and hopefully its stellar sound quality as well. Below is a video comparing the P3 to its predecessor.
Samsung's PMPs have been pretty solid lately, so I'm definitely looking forward to this one. But the touchscreen still doesn't look as responsive as the iPod touch, and that desktop interface looks like it could get awfully cluttered. Still, it's been an unexciting year for PMPs, so here's hoping the P3 lives up to its potential. [PMP Today]