While doing some routine research on mobile electronics and GPS software, I stumbled across this little nugget of wisdom:

If you have a GM vehicle with OnStar, you can hack into the OEM GPS unit/antenna that feeds OnStar data, and with a simple converter cable it can be used with any computer and GPS software!

Here's the how-to:
http://members.cox.net/onstar/

I will give this a shot and post updates as I go. The information is a few years old, but there's several sites with info on how to do this, so stay tuned!

UPDATE!

After doing more research, I discovered that the Onstar systems GM used over the years have changed a few times. In 2004 they started using digital GPS units instead of the old analog ones, and to my knowledge, nobody's been able to hack one of those yet. The older ones use a Motorola chip, and the newer ones use a Sony digital GPS chip. However, it's not definite when the crossover happened, so you'd have to crack the unit open to see what's actually in there to find out. I'm actually kinda bummed about that.

However, for those of you with pre-'04 vehicles equipped with Onstar, this should work for you!

As for my car, the antenna is still there, and adapters are available to use it with standard GPS recievers... I'll do a little more digging and see what I can find!