Possible To Use Ipad As A Portable Charger Or Power Bank?

I vaguely recall reading about this a year or two ago (maybe here, maybe somewhere else), but now can't find anything about it.

Basically, my iPad's battery lasts forever, while my phone's does not. I would like to be able to 'borrow' a charge from the iPad in a pinch.

IIRC, under the previous-gen iPads, you could buy an Apple 30-pin-to-USB adapter (like this) and then plug your charger into it.

Then the spice (I mean, 'electrons') would flow until they balanced out at some point.

Does this sound familiar to anyone? Is this un-possible? Any assistance would be greatly appreciated.