Hopefully the SDK will let you do lot's of nifty stuff like pick who you want to play against from the contacts or detect gestures. Plus the app can work offline, the major downside to iPhone/iPod Touch apps.
i'm not sure how you could pick a network player from your list of contacts. you need to establish a tcp/ip connection; how do you go from a phone number to an ip address?
my idea was to use udp broadcasts to detect other people on your local network who are running copies of the game. if you want to play against somebody who's not on your local network, you'd have to enter a hostname or ip address.
Or have a centralized server, a la Xbox Live, Battle.net, etc. If users of the game "sign in" to the server, than anyone can connect to anyone.
Plus, your phone's contacts typically have iChat contacts associated with them, so if done well, the system could match up based on phone number and/or ichat screen name, and be able to find mobile and desktop clients of your friends.
