ESA's Rosetta team made an announcement yesterday in the form of a tweet from the Rosetta spacecraft.
ESA Rosetta Mission
@ESA_Rosetta
My power is decreasing so far from the Sun I can no longer listen for @philae2014:
Today at 11.00 UTC, they will switch off the Electrical Support System (ESS) on Rosetta, which was used to communicate with the lander Philae. There's been no communication with Philae for a year now. Switching off this system will save energy for Rosetta's final scientific endeavors before she joins Philae on the comet on September 30.
Here is a nice little video (under a minute) from New Scientist that tells the story:
Bye Bye Philae