Finally got it working. All it needed was a 2+ GB SD card and not only 1 GB, which resulted in a restart-loop with error messages not far from “Something is broken! Panic!”.
Next on my agenda – hook up all of my 5 RPis and form a “supercomputer”!
Source: http://bendyworks.com/geekville/lab_projects/2012/11/getting-plan-9-running-on-the-raspberry-pi
Also interesting: http://plan9.bell-labs.com/sources/contrib/miller/
To get you started: http://www.quanstro.net/newbie-guide.pdf
And by the way – the user experience is ground-breaking and paradigm-shifting. The scrolling wheel works in an opposite direction! (+ some other mind blowing things)