I have implemented multiplayer over IP for Diesel Racer 2.
Currently you can only play 2 players(1vs1).
The idea is that you could challange your friend directly using WiFi Direct, but you could connect over IP so it could be any kind of connection that allows to accept connections over IP.
Here is a demonstration of the multiplayer on an earlier stage of development:
What do you think of multiplayer over IP? Would you play against friends IRL with two phones?
If you would like to support me, now it's a good time to buy a copy on OUYA(if you have one).
I almost have the required quota for getting paid, but I didn't reach there yet.
Take note that OUYA was purchased by Razer but I am going to support the Razer box as well.