Analog Inputs on Arduino
On most microcontrollers that have special functions for their I/O, like analog in, but also PWM, UART or I2C, these pins can also be used as General I/O (digital). The Atmel AVR is no exception. You'll have to check the datasheet to see which register(s) enable/disable the special functions.  



This is a detail of the pinout of the ATMega328, which is apparently used in the Arduino Uno (thanks bjthom). You can see that pin24, which is used as ADC1 input, also is used as port PC1 (bit 1 of port C).