> Surely they'd spin up as soon as the BIOS started probing the bus to init the hardware, long before the kernel was running, or they'd be infamous for being "the HDDs you cannot boot an OS from"...

Depends on the market they sell into, might not be a big deal for enterprise drives that you don't expect to boot from; especially if you're attaching them to a controller that you also don't expect to boot from. I had some giant quad processor pentium II beast after it was retired, and the BIOS could not boot from the fancy drive array at all; and even if it could, the drives were on a staggered startup system, so it would have had to wait forever for that.