I am following a few runners at Boston this year, Bob and Warren that I met through the local running store, as well as Steve from the Phidippidations podcast.
Every runner has a chip on their shoe, and when they pass over mats along the course, their split times are recorded. When I ran the Marine Corps Marathon in 2006, they had a system where a text message could be sent to a cell phone. My wife and friends could follow my progress as I made my way around the city. Although the last split didn't get sent out, it was a great idea and gave them idea when I might be finishing.
This year the Boston Marathon is using Twitter. I just received the message: "At 11:34 AM: Half-Marathon Athlete Alert. Bob Harrison. Time 1:32:44, Pace 7:04." Warren is about six minutes ahead of Bob.
Steve is more of a middle of the pack runner. In addition to the splits being sent out, he is also updating his personal progress on Twitter. He has a microphone clipped to his shirt and is recording his marathon for his podcast. Occassionally he uses the mic to call his son to update how he is doing. His son then types it out and posts it on Twitter and his 1,073 followers get the update.
"At 12:06 PM: 30km Athlete Alert. Warren McAndrew. Time 2:05:11, Pace 6:42"