I have run into that error on a number of occasions, mostly for different reasons.
Your devalias for "cdrom" might not match the scsi device for your cdrom drive. Try this:
1) From the 'ok' prompt, type 'probe-scsi-all' 2) Make note of the address of the CDROM 3) type 'nvalias cdrom2 /pci@8,600000/scsi@6,1/disk' substituting in the correct address for YOUR cdrom drive. 4) 'nvstore' to save the changes 5) 'boot cdrom' ... does this boot now?
I've also gotten that error accidentally booting an x86 CDROM in a Sparc box (doh!). Additionally, some Sun machines that came with DVD-ROM drives had bad firmware that prevented them from booting off of certain discs. If your drive is affected, I think you actually need to be running Solaris to install the patch! It is probably worthwhile for you to buy a 9 or 18 GB drive in a sled off of eBay to have Solaris on so that you can patch firmware when needed.