The JoeyBra, designed by two students from the University of Washington, includes a discreet holster for storing valuable things like credit cards, extra cash or, say, an iPhone.
Certainly, carrying a bag or wearing something with pockets isn't always possible, but this might not be the most convenient way to store something you're going to be taking out every 5 minutes (unless you're wearing a strapless dress like one of JoeyBra's co-founders does in the demo video).
JoeyBra is currently seeking Kickstarter funding, and you can get your hands on one for a pledge of $30.
[technabob.]