I can' think of a device that has it's i2c address written on it, unlike, say,
the Dallas Semi 1-wire devices. The device's i2c address is more than
likely stated in the spec sheet for the device. Sometimes all the bits are
specified in the device, making it difficult to have multiples of those
devices on the same bus. Other times there are address pins on the
device that you wire to Vcc and/or ground to set the device's address.
Many devices have a combination of the two -- some bits set in hardware,
the lower bits decided by how the address pins are wired.
Ooops, almost forgot. I2C devices that you hoook up to an Arduino will
pretty much be considered "slaves" with the Arduino being the master
of the bus.