We also experienced downtime in several CRM Online organizations today in the EMEA region, starting from around 9:00 AM UTC. 2 hours later they appear to be functional again. This issue affected organizations on both the old CTP platform (with Windows Live ID authentication) as well as newer Office 365 environments, but some organizations remained functional throughout the period.
I haven't seen any official updates regarding the issue from MS support yet, also I didn't get a chance to open a support ticket before the problem was fixed.