Had a brief problem this morning with one of our internal Xen VPS boxes, this one resulting in our own website being unavailable for approximately 20 minutes. Ultimately it was determined that the Xen DomU that runs our site was unresponsive (and was not coming back online properly after shutting down that single DomU), and we had to reboot the entire VPS machine after applying some Xen updates to be safe. No shared linux web hosting clients would have been affected by this (except for a moment or two where ns3 was offline while the VPS rebooted, and ns3 going offline momentarily isn't a problem, as ns4 is there to take up the workload).
We are currently running a series of Xen updates across all DomUs on the affected machine, which may result in an additional moments downtime while each one reboots into the newer kernels. Again, no client sites should be affected, just our own.