Just launched in LA, WillCall is the next app you're gonna download to your phone. Why? Because then your phone will alert you to concerts in the area, which you can buy tickets to using a stored credit card number. Then, it'll let your friends know you're going, allowing them to decide whether they want to join you/avoid you.
It gets even better: once you're at the show, you can buy drinks at the bar just by telling 'em your name, so long as they're set up through the venue's system. This is a key bonus in the not-wasting-time-going-back-to-a-bar-full-of-regret-to-sign-your-tab-and-get-your-card-the-next-day part of your life.