Just thought I’d report back to say I installed Debian Squeeze on the Mac mini three days ago without any problems, either with booting from the DVD drive or yaboot. It was all pretty much automatic. The CD booted first time and the initial yaboot screen tells you to simply type “install” to start the process. As regards partitioning, I opted for “Guided - Use entire disk” and accepted the suggested layout, which suited my needs.
It has all been running quite nicely so far, with one exception. It seems that Squeeze on a powerpc is no more able to run Flashplayer than Mac OS X Panther 10.3. So no viewing of BBC News videos or YouTube on the mini unfortunately. For YouTube I was advised on the Debian User Forums to try Minitube, which I have installed, but it is not very satisfactory as the videos are jerky and the sound not much better. I think I shall just need to accept that there is limited support for powerpc now. But at least I am putting the machine to some use, which is better than having it stored away in a cupboard doing nothing.