After much arguing, here is how you can set a dual boot system that is slightly less painful than having to type a giant string at the smartfirmware prompt:

Use the diagnostic boot-parameters! Assuming MorphOS is your main system, do not mess with boot-file, or boot-device at all. To put PegXLin as the *secondary* boot system do the following at the smartfirmware prompt:

setenv diag-device /pci/ide/disk@0,0:0
(this should be the drive that PegXLin is on)

setenv diag-file pegxlin root=/dev/(your linux partition) video=radeon
(use the parameters as given in the README on the PegXLin CD.

Now, to switch back and forth between PegXLin and MorphOS, you simply need to change the "use-diag?" variable. If you set it to true, it will boot pegxlin (even after a shutdown). If you set it to false, it will boot to MorphOS (even after a shutdown). The command to change it is:

setenv use-diag? true
or
setenv use-diag? false

This is particularly great if you use Linux through the week for work, then MorphOS on the weekends for fun (or vice-versa). Anyway, it's what I use.