According to a post on Amiga.org the MacMini port of MorphOS is progressing fine but is lacking drivers for sound or network.
You might remember earlier rumours about the rumoured AmigaOS4 port to the MacMini which leaked on the net. It has now been confirmed that OS4 for the MacMini will never happen so it is nice to know that you will at least probably be able to run an Amiga like operating system on an older PPC MacMini.
Just a suggestion, you should stock up on MacMinis now because when MorphOS will be released for it you can be sure price will rocket to the sky immediately.