wolfden, sorry to inform you, but my SL version is x86. I don't have any 64bit computers.
DontPanic, thanks for sharing your experience. I just might unmerge makedev and re-emerge baselayout2.
pajamabama, it seems we haven't come to the real answer to this (yet). Rolling baselayout back will make the xdm load again, but it seems no matter which baselayout version you have installed, portage will say it's conflicted with makedev. This probably means, it's either baselayout or makedev on your system - not both, although you get both with a fresh Sabayon installation. I'm not an expert by any means, but, the way I see it, when two conflicting packages are installed, they will not work properly.
Now for another problem. I've noticed that, since I've re-emerged the old baselayout (=baselayout-1.12.9-r2) and rebooted (while leaving makedev alone), my wireless doesn't work. I can see it in lsmod, I can see it in lspcim, but iwconfig and knetworkmanager see no wireless networks (and I know there should be at least two - mine and neighbours'). Is this a coincidence or could there be a connection?
And, more important, has anyone had a similar experience? How to resolve this?
edit: after removing makedev and emerging baselayout, the wireless is seemingly working again, but xdm fails to load again, and again with the same errors.