It is indeed possible. As the EFI (replacement of the BIOS) of the mac mini seems to have a fallback for default partition tables in case no GPT partition table is found - you can just delete the GPT signature as described in the related FAQ - create a new partition and install linux as usual.
Kubuntu 7.04 runs very fine on my little machine here.

NOTE: As it is also possible to run windows XP on that Box - make sure you first create the Bootcamp Driver CD - before repartitioning the harddisk. The BootCamp Image requires MAC-OS X installed as only OS to create a DriverCD for Windows XP SP2 Prefessional.