I think that "processor architecture" is far less
important than "machine architecture".  While the
Amiga, HP300, and Mac68K architectures are all based
on the 68K series processors, having your OS running
on the 68K series Mac's does not have much relevance
to the Amiga or the HP300 (vastly different I/O arch).

Not sure what the Debian stuff is like, but looking at
"www.linux.org" hardware-ports project list, I notice
that Linux/Decstation boots "multiuser on serial
console" but doesn't seem to have framebuffer support.
Interestingly, Linux/MIPS says that "a port to R2000
and R3000 processors is in progress (hmm, do they talk
to the Linux/Decstation people???), but moves at a
snails pace".  Linux/SH3 boots the kernel and prints
"Hello World" recursively from its screen shots, and
Linux/Vax now boots to a shell, but only runs on the
VaxStation 3100/4000 series boxen.

By far, it seems to me that NetBSD is *far* more mature
on these platforms.  Not that they haven't made good
progress...  I do notice, also, that many of the "ports"
projects seem to be on the 2.2 kernel.

I wonder, from my occasional browsing of the Linux ports
info, just how much cross-communication there is between
these projects.  One of the things I find important here
on the NetBSD boards is that kernel changes (such as the
SMP code) is thought out across *all* the SMP arch's and
is not done in a vacuum by one group.  And a kernel 
change in -current is immediately commented on and fixed
if it breaks one architecture but works on others.