The VM should always pass through proper fsync() semantics, so probably not that layer. But they mention battery-backed raid controllers, and it would be appropriate for fsync() to complete after the data has gotten into battery-backed memory, but before it gets to the actual disks.
(Okay Linode, you've finally done it. I'll get rid of my oddball $30/mo plan sometime soon. That extra $10 used to get me a necessary 180MB of extra RAM!)