The new model(3000 series) comes with a brighter screen than previous models and a built in microphone(which has limited uses so far~Skype and maybe some multiplayer games I think)
It also has newer security measures built in and a different motherboard from older models which have so far been hacked to allow downgrading of firmware, adding of custom firmware and homebrew to the PSP 3000 series. 3000s are unable to be booted from a pandora battery. I dont know much about hacking PSPs, but thats bad for hackers. For Sony its good though.
Actually, its kinda bad so far because there have been lots of complaints of the screen display not being up to scratch. Lots of errors have been reported from what I've heard. No solution from Sony for this, however.
On the plus side, Playstation Network is compatible with PSPs with the latest firmware now. I'm new to Sony products, but thats more or less the equivalent of xbox live downloads from what I can tell. Hopefully they'll make lots of games available for download quickly.
Need to get me some more games for the PSP...