My Ricoh DVD/CDR combo has been happily burning off discs for over 2 years from my box, currently RH8. Last weekend I got to the last of a batch of fedora isos though and got the following -

cdrecord -v speed=4 dev=0,0 yarrow-i386-disc3.iso
Cdrecord 2.0 (i686-pc-linux-gnu) Copyright (C) 1995-2002 J?rg Schilling
TOC Type: 1 = CD-ROM
scsidev: '0,0'
scsibus: 0 target: 0 lun: 0
cdrecord: No such device. Cannot open '/dev/pg0'. Cannot open SCSI driver.
cdrecord: For possible targets try 'cdrecord -scanbus'. Make sure you are root.
cdrecord: For possible transport specifiers try 'cdrecord dev=help'.

and cdrecord -scanbus returns the same message.

Seems to read CDs fine, is there any other way of testing whether it is the hardware that has failed or not?

I think i got the same problem last time , and it was the scsi generic ( sg ) being not loaded
try lsmod and see if sg is loaded. if not you can try modprobe sg

Looks like the scsi emulation is not loaded.
you need to add this line to /etc/grub.conf: hdd=ide-scsi

Here is an exaple on a RedHat system:
To add scsi emulation
#boot=/dev/hda
default=1
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
title Red Hat Linux (2.4.20-8)
root (hd0,0)
kernel /vmlinuz-2.4.20-8 ro hdd=ide-scsi root=LABEL=/
initrd /initrd-2.4.20-8.img

then try the 'cdrecord -scanbus' command again

Nah, I think it's a hardware problem, it's not working in a windows machine I put it in either