You could look into hdparm or your could poke through the /proc entries for the hard drives.

/proc/ide/ide0/hdb/ for instance will have lots of info on the second drive on your first IDE channel. /proc/ide3/hdg/model will have the model number of the first drive on your 4th IDE channel.

I'm not entirely sure how unique it is, but I think it's partly from the serial number, so /proc/ide/ide0/hda/identify should have some identifying information for the first IDE drive on your first IDE channel. Hopefully it's universally unique.