Originally posted by another user
Don't make any plans for a day of Xbox Live fun and happiness next Tuesday. Major Nelson has posted on his blog that Microsoft will be taking Xbox Live down for maintenance on June 16 at 12:01AM PST for "up to 24 hours," although any World of Warcraft player knows not to count on those sorts of estimates. In addition to Live being down, the My Xbox section of Xbox.com will be down and the forums will be put in read-only mode.
As is usually the case with maintenance on Live, this won't be to implement any new features and is simply scheduled maintenance to prepare Live for some of the features announced at E3. Luckily none of this will inhibit your ability to play the newly released Prototype offline.