WWDC 2008 News: Loopt shows off new app for the iPhone
This video refers to the iPhone a lot but Loopt is available on over 100 gps enabled phones. I just put it on my G1 and it is pretty cool. It does Twitter and Facebook integration and allows you to set all permissions (who can see you, what location to share, etc.). If you have a gps phone you should really check it out!