I didnt work with phpundercontrol yet and from what I read so far, I dont see whether you can use it to control/CI multiple projects that are "living" on different servers.
Do I really have to run phpundercontrol on each of the production servers? Cant I run a centralized phpundercontrol "to rule them all" ?
Helpful answers appreciated
EOF
Yes, you can run a centralized phpundercontrol "to rule them all". Like already pointed out in the comments, you should have a dedicated server that simply fetches your various projects from your Version Control System and integrates/builds them. Unless these builds succeed, you should not deploy any of those projects to your production site.
On a sidenote, most people I know dont use PHPUnderControl anymore, but have moved to use http://jenkins-php.org, which is much easier to setup and configure.