Arriving with the update is a dedicated ESPN hub for "more than 3,500 live and on-demand global sporting events from ESPN3.com" and Zune Music, a feature that--when coupled with the premium Zune Pass--gives players "unlimited, on demand streaming of millions of songs."
Some new content, is only available in select regions. Both aforementioned additions require an Xbox Live Gold subscription.
As previously reported, the update will also include improved voice chat quality over Xbox Live, a Netflix application for Xbox 360 consoles in Canada, and the ability to search through the complete Netflix library, directly from the console.
For a complete list of changes, head over to Major Nelson's blog. Also, don't bother turning your console on and off to force the update, Major says that won't work.