Copied from the OnStar website:
Q. What different types of equipment do OnStar-equipped vehicles have? A. All OnStar-equipped vehicles have one of three types of equipment:
Analog-Only: OnStar-equipped vehicles with analog-only equipment were designed to operate only on the analog wireless network and cannot be upgraded for digital network compatibility. Vehicles with this equipment will no longer be able to receive OnStar services beginning January 1, 2008. At that time, service will be available only through dual-mode (analog/digital) equipment. Analog-only vehicles cannot be upgraded to digital equipment.
Analog/Digital-Ready: OnStar-equipped vehicles with analog/digital-ready equipment operate on the analog wireless network, but were designed to be upgraded to dual-mode (analog/digital) equipment when available for that vehicle. Beginning January 1, 2008, OnStar service will not be available on these vehicles unless the OnStar equipment has been upgraded to dual-mode (analog/digital) equipment.
Dual-Mode (Analog/Digital): OnStar-equipped vehicles with dual-mode (analog/digital) equipment operate on both the analog and digital wireless networks and will not require an upgrade in connection with the wireless industry's transition to the digital network.