Downtime is a rarity with Office Live, but an announcement has just gone out that on Saturday September 12th 2009 and Saturday September 19th 2009, there will be some periods of planned outage.
All customers will be sent an email to this effect later this week.
What this actually means is that for your Office live website, there will be three hours on either the 12th or 19th September when you will not be able to do any site maintenance and visitors to your site will see a message detailing the outage. You will still be able to log in and check email during this time.
This upgrade work is designed to further improve the performance, reliability, and stability of all Office Live websites.
Read the official posting here : http://ask.officelive.com/smallbusiness/blogs/team/archive/2009/09/01/brief-site-downtime-planned-to-accommodate-a-server-upgrade.aspx