Microsoft will start to make the new Xbox 360 Dashboard update available starting at 2 am Pacific Time today. (That is in 30 min. from now)

The Spring Xbox 360 Dashboard has 125 new features and enhancements. Microsoft's Major Nelson already previewed some of the improvements in a video report a couple of days ago.
The major improvements include the download manager, boot to dashboard, new music player features, improvements of the DVD player and system improvements.

The complete feature list is available on xbox.com.
More details about the Xbox 360 Spring release can be found on Xbox.com. See also Major Nelson's post about the update process.

Xbox 360 Deals.