Looks like the outage is over: about an hour and a half ago, the Linode status blog (http://status.linode.com) announced that the upstream provider was going to start gradually restoring access to their Atlanta datacenter (and that includes us).
 And we're back down again. From Linode:
The attacks against Linode have been going on since the 26th, however, and show no signs of stopping: I'm cautiously optimistic for now, but until I have some reason to believe that the attacker(s) will stop, I have no reason to believe that we won't have any more downtime.
In the meantime, I'll be working on two things: finishing the work on the XLS export feature (which should be ready to go in the next release) and building some redundancy into my setup.
The bottom line is that this is at least partially my fault, and I am going to do a better job holding up my end going forward.