According to the ESA team: The illuminated crescent is centered roughly around the South Pole (South at the bottom of the image). The outline of Antarctica is visible under the clouds that form the striking south-polar vortex. Pack ice in front of the coastline with its strong spectacular reflection is the cause for the very bright spots on the image.
[update]Rosetta has been taking regular pictures during the course of its Earth pass, and ESA will be releasing them as a video shortly. Too cool!
Rosetta will make its closest approach on Friday at 08:45 CET.
Image credit: ESA © 2009 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA