I ran into something interesting today you may not be aware of, I wasn't.
I tried to start a macbook pro off a service diagnostic disc. Normally I put in the disc and hold "D" (for intels) to boot off the special discs I get from apple. They contain an advanced version of the hardware test and work on a number of machines instead of just one model.
Anyway, it booted to apple hardware test, for this model. (AHT 172 iirc) Turns out it's on the HARD DRIVE. (maybe) And holding "D" down prefers a diagnostic image on the hard drive over that in the optical drive.
Holding Option at boot was required to select the service diagnostic for testing. The hardware test does not show up in the startup picker, only the service diagnostic disc and the hard drive.
I've seen this once before on a new imac. Apparently apple is placing the hardware test image on a hidden partition on the hard drive somehow... possibly stuffing it in with the drivers? Or maybe the diagnostic image is in the ROM somewhere? (they're not very big, it COULD be made to fit I think)
Note: The AHT discs I've seen all tell you to hold down "A" to startup off the hardware test on the restore disk. I was holding "D", not "A", and normally those are not interchangeable.