Integrate with Glympse!
You offer Foursquare check-in which is great! http://glympse.com/ is a great app that I use often to show people I'm on the way. At any time, they can see where I am and what my ETA is (if destination is set). I see you have MEET UP, which shows current location or destination, but Glympse sends constant updates showing where you are. It's available in Garmin Street Pilot and Navigon now (http://www.glympse.com/partners)!
Martin Albers commented
+3. Waze's sharing of ETA is good and reliable, but does not share anything about the sender's position. Glympse's sharing of map / route is great, but their route calculation and ETA s…cks. Best combine both into a really great product.