Yes, OnStar uses standard cell service through Verizon. The other major issue is that by 2008, OnStar has to go all digital.
This means that unless you can reach a digital cell site, you're stuck.
I've been in the boonies where Verizon, Cingular, Alltel, T-Mobile, and even analog doesn't get you anywhere, but Satellite phones will.
For more information, check out this FAQ:
http://www.onstar.com/us_english/jsp...iew=tech_equip