/etc/init.d/zope2.8 restart didn’t work on my Ubuntu Server 5.10. It turns out that it’s using Ubuntu’s homebrewn dzhandle script to handle plone instances. You are supposed to create instances using dzhandle. If you do, there’ll be a file named debian_policy in the etc directory of the instance, and if that file exists, dzhandle will recognize the instance and happily restart it.