Devotec has created a mobile phone charger, and this isn’t really unique, because a lot of companies do this. In fact, I’m a little tired of putting a link to Mophie or whatever company I can think of. This charger, simply known as “Fuel” is the smallest. (See the quarter put there specifically for comparison?)
Fuel only has a 5C lithium-ion battery, rated at 220 mAh. As you imagined, it can only give 20 and 30 minutes, as it attaches via micro-USB. There is an on and off switch for discharging power, and it is made for the keychain not to mention emergencies where you really need that power.
Yes, this is a Kickstarter project, and yes, it has almost made its funding goals. You can find out more here.