I found this regarding the Live downtime yesterday.
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Reminder: Xbox Live Maintenance Starts Shortly
Just firing a reminder out to everyone that in just under 2 hours (12:01am PST, 8:01am GMT) various aspects of the Xbox Live service will be down for up to 24 hours for maintenance.
So that means Xbox Live and the My Xbox area of Xbox.com will both be unavailable, whilst the Xbox.com forums will be read only. Who needs Xbox.com anyway when you have x360a. *wink wink* *nudge nudge*
With regard to what it is ... Major Nelson says "scheduled maintenance," but if you wanted to try and connect the dots further, the Zune service will also be down for the same period. Whether MS are doing all their spring cleaning together, who knows, but I would say, it doesn't take a genius to see what they're preparing for.
Weirdly enough as well, Twitter will be unaccessible for an hour of downtime from 2PM PST (10pm GMT) today. Please don't tell me that's coincidence too.
Notice I said preparing though, because Major points out that "you won’t see anything different on your console" when they fire the service back up on Wednesday.
Looks like offline achievement whoring today then if you need a gaming fix.