A few friends and I have been using a program called Chirp (recently changed name from TruePTT). This is basically a person to person Push to Talk app. We broke off into two groups tonight and hit the streets of St. Louis armed with Chirp and Zombie, Run. Hilarity did ensue. Watching a group of friends run, two streets over, while screaming, "Zombies on our 6, we're working our way over to your block!!" through their phone was one of the most entertaining things I've done in a while.
If you're interested in PTT, I'd definitely check out Chirp. In the market it's available for $1.99 for the full version, and the Lite version seems to have disappeared for the time being.