Road Trip shows a couple planning a cross-country trip, and using Siri to learn the best route, to find the best barbecue and find out about rodeos along the way, to find out where they are, to learn how big the Grand Canyon is, to find gas stations within walking distance (d'oh!), to learn what the constellation Orion looks like, and to remind them to do it all again.
Rock God shows a young man planning a music carrier, and using Siri to find out where to buy a guitar, how to play certain songs, creating lists of potential band names, to send messages to the ladies about when and where his band will be practicing, and to call him "Rock God".
With Siri still in beta, there have been arguments raised against Apple making it the staple feature of their iPhone 4S advertising, saying it doesn't work in real life anywhere nearly as well. However, Siri is also the most impressive, most demonstrable, most advertise-able feature of the new iPhone.