The major differences are the buildin mic and newer LCD technology.  Oh, and the "hottest feature: reduced battery life", according to Jim Sterling at Destructoid.com.  From the pictures shown below, whether the new LCD would justify the shorten battery life (20-30 min) is up for you to decide.  I personally prefer the PSP-3000 model.

Front view (up 2000; down 3000):

Back view(up 3000; down 2000):

According to Famitsu, the edges of PSP-3000 fit your hands better when holding it.

LCD Spec (2000 vs 3000):
Size                        4.3 inch (both)
Resolution               480 X272 (both)
NTSC color gamut    53%        85%
Contrast ratio          400:1      2000:1
Response time         30 ms      8 ms
Anti-glare                no           yes

Some pictures (up 3000; down 2000)

sources:
http://www.famitsu.com/game/news/1217756_1124.html
http://gnn.gamer.com.tw/9/32249.html
http://www.jp.playstation.com/info/release/nr_20080902_psp3000.html

Perfect for movies , i'll buy one when it gets cheaper.

The screen looks much better in the newer version

Yeah, the wider color gamut and contrast ratio help it tons.

I may get a new one this Christmas.

Will games take advantage of the extended colour gamut?

i would assume games with a lot of pre-rendered CGI cut scenes will benefit more from the wider color gamut, for example, crisis core: ffvii.

i am more intrigued by the 8 ms response time. don't know how much difference the shorter response time gonna make for the game play.

The screens quality is far superior and the 2x faster response time is great to :D cant wait :D

Can someone show me ghosting? I don't think I have seen it in action before.

Nevermind, I see it.

But I don't think I've seen it before.

Oh well, for those that have suffered from it, it's apparently helping.