Based on the many forum messages about activation, it appears that no one
outside of an appropriate Microsoft group really knows the details of the
activation process.
It is likely that OEM installations involve some Microsoft operating
system related customization of the BIOS, but I agree that this may not be
correct.
A number of people have independently confirmed that a BIOS upgrade may
require a re-activation. The BIOS changes that might trigger a
re-activation are unknown to me.
On a slightly different subject, we have done a "bare metal" (Microsoft's
definition, not mine) restore after using Windows Server 2007's "Backup",
which, incidentally, is a great program. I am ashamed to say that I don't
remember if this restore required an activation. I was so pleased that it
worked flawlessly that I didn't pay attention to a possible activation
request.
An interesting question is whether or not such a restore would work if
there had been a significant hardware change; for example, replacing a
motherboard. I do know that that you must have the same number of hard
drives and that each of these must have a capacity equal to or greater
that that of the original drives. There may be some additional technical
limitations.