Great I found it. Seems they had the unit in the trunks of early 2000 grandams but decided to wedge it way deep into the dash behind the glove compartment.
I read up a few places where people had taken out their Analog OnStar boxes (i think these are standard in 2005's but Digitals in 2006), soldered a serial cable and using Motorola OnCore software to change it to NMEA mode were able to use the gps transmitter connected to a laptop with Streets and Trips or any other program that supports a GPS transmitter.
Since onstar will discontinue use of the Analog boxes in 2007; they will replace the analog box with a digital one for "Free" (Free = signing up for at least 3 straight years of service at the time of the swap). One of the only bonuses i can see with the Digital boxes is that if you are a Verizon customer, you can add the OnStar phone for 10 dollars to your "family plan" ... phones that will all pull from your pool of minutes.
I never used onstar other than setting it up and don't really see the bonus of paying 225 bucks a year for something AAA can do for 50 (even though you actually have to pick up the phone and call them). Regardless, after the free year is up, why not take advantage of the really expensive GPS stuff in your car (you paid for it!)
An Online Guide to getting to the RS232 Serial Data from your OnStar GPS in NMEA format.: http://members.cox.net/onstar/
I haven't done this, has anyone tried this experiment?