I'm currently running a Mini as a cheap KVM virtual host server - because you can squeeze two into a single RU, they're quiet, and use relatively little power.
The secret is to install using the 11.10 'alternate' disk, located here, which will get around the biggest issue which is botoing under Apple's UEFI. My next recommendation is to keep a small OS X partition on the disk, and ensure that you run a system update piror to the Ubuntu install, to ensure that your firmware is as up to date as possible. You may need that OS X partition in future to install other firmware updates from Apple.
There's a whole stack of step-by-step instructions (that I read and ignored) on the Ubuntu community documentation wiki here on what you want to do.
- Yes - if you're not afraid to get your hands dirty;
- You can either beg/borrow/steal an external CD/DVD drive (which is my preferred option), or install from a USB stick instructions found here (I did ask if you weren't afraid to get your hands dirty!);
- With Ubuntu 11.10, the only issues with drivers I've had are DRM (direct rendering), which won't affect you as you're not running a GUI.
Finally, to the comment that it's 'not supported' - Linux was never supported on a whole stack of architectures that it was ported to originally - if we all had the 'not supported' philosophy, we'd not have the freedom of Linux on so many architectures now.