Microsoft has posted Details about the Spring 2007 Dashboard update on Xbox.com, rumored to be released on May 7th.  One of the big additions is the integration of Windows Live Messenger,  a dedicated blade for Marketplace, tweaks to the friends list and achievements including the achievement message that pops up will now display the point value and achievement name,  also the name of the game in the disk drive will now be displayed on the dashboard.

There is a brief video (Xbox 360 Spring 2007 Dashboard Update Video) on Youtube with some of the features and

They will also be releasing a QWERTY text-input device that will attach to the 360 controller for text messages.  It should be available this summer.