It's smaller, has more ports, runs quieter and costs the same: $300. It has a 250GB hard drive and built-in 802.11n Wi-Fi. They're shipping the new Xbox 360 to retailers today.
Update: Here are some detailed specs. One of the most interesting details is that this new system has been in development for, give-or-take, two years.
Update 2: Some more new details:
The controller is slightly changed, with a chrome Xbox button, the Xbox logo on the back, and squared off shoulder buttons, which don't change gameplay but give it a more angular aesthetic feel. The angularity is also evident in the Xbox 360 itself, which Microsoft said that they updated to match the look of modern AV equipment, which are usually black and glossy and chrome.
To transfer your hard drive from your old console, you can either use a standard old 16GB USB drive and do it chunk by chunk, or you can use the old hard drive transfer cable (it works with the new internal hard drive).