Like what happened in Magnolia a few years ago, and what happened with Digg a year ago or so.
What preventative measures are in place? As in, redundant servers and backups of old data?
The data is made available in bimonthly data dumps under the CC-by-SA license. There are some sites (at least a dozen I think) that host data from the dumps in different forms, so I think even in a major disaster most of the data could be recovered. Feel free to host a mirror yourself though as long as you comply with the above license.
Also, the data center moved to NY back in October.