On a more practical note, while making a bootable CD isn't
   hard, I gather that making a bootable CD that boots on different
   systems is much harder, and those who invest in CD production don't
   relish the idea of N different CDs for the various architectures that
   have any ability to boot from CD.


well, from what i've managed to find out so far:

- it appears possible to put "el torito" support for any platform in
  an iso image, which means that all of these ports should be able
  to boot from any cd that contains a bootable image.  so far i only
  know of i386 and powerpc el torito support.

- sparc & alpha can not share bootable cdroms, due to conflicting
  disklabel/boot sector issues.

- sparc & alpha booting is _separate_ from the iso image, allowing
  them to be shared with, eg, el torito booting.

- the ultrasparc _may_ be able to be shared with the alpha, as it
  actually reads the iso image to work out where to boot from, but
  i have not investigated this at all.