Ooh, I think I've solved it.
I did a bit more Googling and came across this thread
(and being a computer engineer, I don't know why I didn't think of this). It says in a post in there that it's more of "I/O" light, rather than just purely disc activity. It talks about it polling the DVD drive to see if a new disc has been inserted. So, I clicked Safely Remove Hardware and "ejected" my laptop's DVD burner. Now the light pulses at a much, much less frequent rate. It was pulsing at a rate of about 1-2 pulses per 2 seconds before, I've just timed in and it's pulsing at about 1 pulse every 5 or 6 seconds, which is more what I was expecting under general use conditions. I guess I didn't notice it because the seeking on this hard disk is very quiet, even in my nice quiet room.
I wonder if it's a bug in Dell's latest BIOS or something (this is a brand spanking new model, so I guess that's possible).
Brb, rebooting to "re-attach" the drive.
EDIT - And sure enough, light goes berserk again. Ejected the drive again and it stops. That's incredibly weird.