Thanks to the excellent support, moral and otherwise, I finally convinced
myself to back up my iMac (several GB over a 10Mbit hub) and reformat the
disk with drive setup. Of course, the 1.5.1_BETA2 install went smoothly
from there (except for the point about missing gm0, but I handled that by
booting an older GENERIC_MD; this method worked fine.)

I couldn't get X (as provided from the BETA2 snapshot sets) to work though,
so I compiled a new kernel with INSECURE and tried X-4.0.1-1.5.

It works - almost !
My display is set to 1024x768,75Hz 16bit in MacOS.

When I start X with startx I get a black screen, unless I put
Modes "1280x1024"
in the Screen section's Display subsections.

If I do that, X comes up with some xterms, but the screen is divided in two
or three repeating bands or rectangular fields, one a couple of inches wide
from the right side.

I can use the mouse, and type in the xterms and move them around, but I
can't stop or kill the X processes to return to the console.

I guess I'm now faced with some "simple" task of getting my XF86Config
right, but again I must confess my incompetence and beg for hints. Having
to reboot each time because the Xserver can't stop gracefully makes
trial-and-error rather painful.

-Lasse