Skylock ($159 to preorder, ships 2015) is a Bluetooth-enabled, solar-powered lock that automatically unlocks when you approach, and it locks automatically when you snap the shackle into the lockbar. If your phone dies, just enter in the numeric code you chose during initial setup manually. If someone attempts to steal the bike, Skylock will send an alert.
Can't wait until 2015 for a keyless U-lock? Check out an alternative, Bitlock, which ships November 2014.