I finally got around to trying this out a few days ago. 

If I set the bootdef_dev variable in the SRM to multiple devices
with a comma between them then the SRM will attempt to boot from one
until it gets to a device that actually works. I configured my test
box to boot from the CD-ROM and then the hard drive, figuring this
would be handy when the machine is turned into a read-only firewall
later.

Pretty slick.
-- 
"A method for inducing cats to exercise consists of directing a beam of
invisible light produced by a hand-held laser apparatus onto the floor ...
in the vicinity of the cat, then moving the laser ... in an irregular way
fascinating to cats,..." -- US patent 5443036, "Method of exercising a cat"