I have built a converter from 12v to 5v
That's enough for RaspberryPi's 5v requirement. IT also gives power for fan with adjustable thermostat (12v) and even active usb hub (in the future). Everything (Raspberry Pi, converter and thermostat) is enclosed in a router-box (I even have space for an additional 2.5" hdd, 3G modem and/or GPS unit ;) )
Such a converter can convert power from a battery pack (for example from 5 or more AA batteries) and SAFELY give 5V for Raspberry (watch out for voltage!). The converter is pretty small (i think 3 x 10 x 1.5cm). But if you are good at soldering and building circuit boards you can do it smaller.
This is the converter I use with a sample schematic.
It works great, you can attach from 7 to 40V from batteries. And we got max 3 amps on 5v side.
If you need to switch between phone charger and battery pack, you must build a switch (when power from charger goes down, the battery-pack power is connected and vice versa). I think that's pretty simple (transistor, capacitor... just few elements). Or do it manually ;)