Clever workaround to not being able to run in the background on your iPhone: Social network service Loopt has a deal that allows them to constantly update your location (presumably using AT&T's servers), even when the app's not running.
It's a limited trial for now with 5,000 slots, and it'll cost $4 a month after 2 free weeks of the service. Loopt says with the new real-time location powers, they can build a "life graph" showing everywhere you've been, so they can deliver personalized recommendations (read: ads) to you. Man, this woulda been sweet with Google Latitude (and probably free, though I don't see Loopt charging for this for too long, honestly). [Loopt]