I've no idea what the "MTU" is, or what it means if it fails. However, yes, they are performing an upgrade of the Xbox Live (and overall Xbox 360) interface. Xbox.com is also down during this 24-hr period. They've been all abuzz about the changes for the past few months. Your "blades" will be gone, and replaced by a more user-friendly interface including a Wii-like avatar. Should be interesting.
In case it matters, we've also been reassured that content we've purchased (themes, gamerpics, etc) will not be removed, and that we will be able to utilize it all somehow with the new interface. *shrugs*
Keeping updated w/Xbox Live (prior to today, of course!)