Question: Can you install MorphOS alongside of an existing OSX installation? The dual-boot guide seems to cover installation from scratch (e.g. install OSX and MorphOS from scratch and have them co-exist)? Thanks.
If you want to do that you need to resize your existing Mac OS X partition (unless of course you have unpartitioned area on the HDD, in which case you can just continue adding the partitions needed by MorphOS and skip installing Mac OS X).
The resize should be possible by booting off the Mac OS X Leopard install DVD, or if you don't have that,
by using Ubuntu.
Once you have enough storage for the MorphOS and MorphOS boot partitions (as described by the pdf) you should be able to proceed to create the partitions needed. Just skip the part where you install Mac OS X (as it already is installed, obviously).
Warning: Resizing a partition is always a hazardous trick. Be sure you've backed up all data you can't afford to lose in case the excrement hits the fan.If this all sounds too complicated you can always back up all data (Time Machine), boot off the install DVD, repartition and restore the backup.