I have a K2, and the new screen with supposedly 50% better contrast tempts me a little.
But then again, I still happen to feel that the K2's screen is already close to perfect... so no, I guess I won't be upgrading.
You may also want to take a look at the "Kindle 3 vs Kindle 2 Comparison" at the following link.
http://ireaderreview.com/2010/07/28/...3-vs-kindle-2/