Yes, Ubuntu Hardy Heron works just great on the Mac mini. Well, at least for the most part. I you have an Apple keyboard you will have to pull of some tricks to get it working 'better'. For example the 'num lock' does not work and will screw up your keyboard usage, I found you can reverse the effect by pressing the 'fn' key, which is located at the 'insert' key location on a PC keyboard.
You can easliy solve this buggerness by enabling this function in the 'keyboard layout' options in the 'preferences' (see screenshot). Another thing is the none-working function keys. Really great, luckily these can be enabled simply by the command;
echo 2 > /sys/module/hid/parameters/pb_fnmode
put this in your '/etc/rc.local' or for your lazyness. The one major drawback is that the sleep function isn't working. It is probably kernel related, as 2.6.24-12 from rc1 worked just fine, but the current 'stable' 2.6.24-16 doesn't. I'll try to figure out which part is responsable later.