Even the traditional hosting methods are not immune to outages. Rackspace, who own, operate and manage the Mosso Cloud Servers, took a wobble yesterday with a brief outage to their main hosting data center due to a power problem. This is the second time in as many weeks, with some losing connectivity depending on which part of their data centre you were on.
It is unclear whether or not their cloud offering was effected but I can say, first hand, as we are running a number of instances on Mosso, that none of our monitoring processes flagged anything. So for at least some of us, we were unaffected.
Whether you are running in a traditional data centre, or within the cloud, it is vitally important you perform your own server monitoring for uptime and accessibility. Even if you just run a simple ping, or utilise a 3rd party service (http://www.ippatrol.co.uk/) then make sure you have a means to which gauge if all is well in your external servers.
Keep an eye on your servers - no one else will.