As Microsoft gears up to bring us a new Xbox Live Experience, Xbox LIVE will be down for 24 hours.
It started at 12:01am and will be online again at 11:59pm PST
The downtime will affect Xbox forums will be read only mode and also My Xbox section and after its back up again.
Expect no new features as the downtime is only for laying the foundation for the Xbox Live Experience this fall
via source
The downtime actually lasted 36 hours: http://www.justuptime.com/reports/951653685dcccd594da21547fe7e63c4
Shame, but up now anyway :)