Apparently Apple has created a signature server, and the device phones home whenever you restore your iPhone or Ipod via Itunes. It checks the device identifier and the firmware version on your phone.
What this implies for i-product users...is that Apple can prevent users from being able to downgrade the OS...case in point with the current 3 gs, those users cannot downgrade to previous versions of the OS, because apple is no longer giving approval or signing lower versions. So even though 3.1 has proved problematic users on the 3gs or who hadn't previously had the signature info stored elsewhere (eg via Cydia) can't use earlier software on their devices.
from TUAW
What this implies for i-product users...is that Apple can prevent users from being able to downgrade the OS...case in point with the current 3 gs, those users cannot downgrade to previous versions of the OS, because apple is no longer giving approval or signing lower versions. So even though 3.1 has proved problematic users on the 3gs or who hadn't previously had the signature info stored elsewhere (eg via Cydia) can't use earlier software on their devices.
from TUAW