This note and images were just sent in from the MARSIS Team - it appears they did in fact receive some nice returns during Sunday's Phobos flyby. The instrument's 40-metre long antenna is usually used to send low-frequency radio waves towards Mars, which are then reflected from any surface they encounter.
The team wrote:
The multi-frequency sounding radar MARSIS successfully observed Phobos during the latest Mars Express science campaign on 9 January 2011.
MARSIS collected two segments of data containing 6000 individual echoes, acquired in 50 sec of operation. The distance from Mars Express to Phobos was in the range 180 to 230 km, using a frequency of 4 MHz.
The ground track covered new areas not explored by previous flybys. From a first analysis of the topography and based on previous experience, the layered appearance of the image, also called 'clutter' is caused by delayed radar reflections from the surrounding Phobos surface shape.
Fig. 1 (scroll down) shows the radargram of the first segment of the flyby. The top white line is the surface signature of Phobos, while the clutter or sub-surface contributions are the bottom ones.