Update:
Xbox 360 Spring Update 2008 rumored

The details about the Xbox 360 Spring update are available. The Spring update will start on May 7th.

We already reported about the Windows Live Messenger service that will be part of the update. This Xbox 360 contains apparently hundreds of new features and enhancements.

Microsoft improved the achievement handling, introduces enhanced family settings features for Xbox Live and a new Xbox Live Marketplace blade lets Xbox Live members access the content they seek.
Updates to Xbox Live Arcade allow owners to quickly see which Xbox Live Arcade games their friends are playing and join in the fun.
See the full list on xbox.com.

Besides the new Windows Live Messenger integration the Xbox 360 Spring Update is more like a maintenance update than a major update with new capabilities.