Since I saw the pre-release photos of the Xbox 360 Slim (or S) a few months ago I've considered upgrading from my old, original release 360. I wasn't one of those crazy people to stand in line for days to get the original on the release date, but I picked one up a few months afterwards.
The new 360 Slim has a nice new look, touch sensitive buttons, quieter operation, less power consumption and an 802.11n wireless port. The problem with upgrading is that those few things I just listed are the only reasons to upgrade. As far as I can tell it isn't any faster, doesn't contain a Blue Ray player and the original can sync with the Kinect without any problems. So why spend the money to upgrade?
Selling your old Xbox 360 on eBay seems to be a loosing proposition considering entire setups with HD DVD players, multiple controllers and games go for less than half of the new premium version.
Luckily if you plan to get a Kinect when it comes out Microsoft has offered an Xbox 360 S bundled with the Kinect for about $50 less than you'd get for buying them separate. In the end that may be what I do. I may opt for the less expensive 4GB version (plenty of space for me) and the Kinect all in one for about $300. Hopefully I can get a little bit of money for my old Xbox 360 Pro but I think I'll keep my old, white, gear to use with the new machine. No reason to run out and buy all new equipment!
Check out the Wired.com review.