http://news.teamxbox.com/xbox/13197/Xbox-360-Spring-Dashboard-Update-Features-Leaked/
Windows Live Messenger interoperability:
Add up to 600 Windows Live Messenger contacts to your Xbox Live's Friends List.
Basic text chat functionality with Windows Live Messenger users. Audio and video might be in the works.
You'll be able to chat with up to 6 friends online while playing games or watching movies.
Your Windows Live Messenger contacts will know which games you are playing, as well as your gamertag.
You'll see your Friends status, whether they are playing a game on Online on Windows Live Messenger.
Xbox Live Marketplace:
Xbox Live Marketplace will now have its own blade.
The XBL Marketplace blade can have its own skin, independent of the dashboard's theme.
The XBL Marketplace blade will change looks depending on such events such a season, a holiday, etc.
Achievements:
When you unlock a new achievement, the pop up will show which Achievement you got and how many points you earned.
Downloads:
Auto-shutdown: When enabled, it turns off your console after 6 hours on inactivity or after all the items in the queue were downloaded.
Low-power downloading mode: it turns off all your fans when you leave the console downloading content.
View partially downloaded movies: self explanatory; you can view parts of a movie that have been downloaded.
Source: TeamXbox
Windows Live Messenger interoperability:
Add up to 600 Windows Live Messenger contacts to your Xbox Live's Friends List.
Basic text chat functionality with Windows Live Messenger users. Audio and video might be in the works.
You'll be able to chat with up to 6 friends online while playing games or watching movies.
Your Windows Live Messenger contacts will know which games you are playing, as well as your gamertag.
You'll see your Friends status, whether they are playing a game on Online on Windows Live Messenger.
Xbox Live Marketplace:
Xbox Live Marketplace will now have its own blade.
The XBL Marketplace blade can have its own skin, independent of the dashboard's theme.
The XBL Marketplace blade will change looks depending on such events such a season, a holiday, etc.
Achievements:
When you unlock a new achievement, the pop up will show which Achievement you got and how many points you earned.
Downloads:
Auto-shutdown: When enabled, it turns off your console after 6 hours on inactivity or after all the items in the queue were downloaded.
Low-power downloading mode: it turns off all your fans when you leave the console downloading content.
View partially downloaded movies: self explanatory; you can view parts of a movie that have been downloaded.
Source: TeamXbox