First of all, this site has not really been updated at all since I first put it up in January 2003. GM has made a lot of changes to their onstar system since then. If you have a 2002 or older model car, the information on this site should still be useful. Otherwise, your best bet is to check out the forum, and see if anyone else has experience with your setup.
If you have visited this site before, you may have encountered the extremely annoying message board hosted by aimoo. This board has so many dreadful popup ads that it is too irritating to use. The worst part about this board is that aimoo deleted all the older messages, so all the good information that was posted a year or two ago about other folks experiences with different GPS units and such has been lost.
For this reason, I have gotten rid of aimoo, and would instead encourage the use of Dave Holst's Onstar Forum at http://www.onstarforum.com.
One comment I often hear is "What's the point? Just go buy a GPS." This is a very good point. But if you like to tinker, and have fairly easy access to the onstar box, it might be fun to mess with it.
If you just want a GPS in your car, you can find one fairly cheap at someplace like http://www.deluoelectronics.com. These things are less than $100, and will work with USB, which is particularly nice since many of today's laptops don't have a serial port.