Just download google maps (app) onto your BB. It will use your GPS (if available) or cell tower to show your location on the map. And it updates in realtime (moving map). Very nice!
Now if only more of my buddies would get a BB I could track them around too!
Edit: and for those trying this out, it updates about once every 10 minutes or so, so it's not like you can see the car driving down the street like a GPS, but it's still really cool.