Hotplug scripts not working
This is, bar none, the error message I dread seeing most. It's got a lot of causes, ranging from the silly to the diabolical. Furthermore, it can strike any class of device -- disk, vif, vkbd are the ones I've seen.
First, to avoid kicking yourself later, check the syntax of your device lines. Myself, I comment them out one at a time and see what happens to the error. It's a dumb troubleshooting technique, but sometimes the old ways are the best.
Try using -x. This makes shell scripts print out commands as they run them -- it's like a constant backtrace. If you stick 'set -x' at the top of, say, /etc/xen/scripts/vif-bridge, output will go into /var/log/xen/xend-hotplug.log.
More to come!