Yanko Design posted a new phone concept on their blog yesterday. Now remember, this is by no means a practical device (currently); it's more of a "wouldn't it be neat of we could make this" kind of idea.
The idea behind the hybrid screen is that you'd use the E-ink for static images and as a low power mode. I'm not sure this will save battery life; the phone CPU draws so much power that I'm not sure you would notice the savings.
The Second Life Mobile Phone Concept works on the premise that we often forget about the battery life of our mobile phones, hence run out of juice, just at the wrong time. As a remedy (rather power-saver-option) the phone incorporates a double display. When in active use the AMOLED screen powers up in all glory and in standby mode the E-ink display gets active. This essentially means that the E-ink uses very little energy and the degree of transparency of the display shows how much battery is remaining. The more transparent the display, the less battery remaining.