Folks, I would advise against this. This was the case with older Ni-Cd and Ni-Mg batteries but the iPhone, as most other newer devices uses a Li-Io or a Li-Pol battery. And for these kinds it's actually akin to death to drain the battery completely. This is why you might notice that the iPhone actually shuts down BEFORE reaching 0 - to have a bit of charge left to keep the battery alive.
Originally Posted by iPhonephreak