The Maylaysian jetliner that went missing has yet to be found. There are many remote sensing satellites, equipped with radars, currently in service. Together, they provide good coverage of the entire Earth. Why is it their data could not be used effectively in tracking down the path of an airplane? If the airplane lands on the ocean, shouldn't we be able to tell from satellite data?
Civilian earth observing SAR satellites do not always operate over empty swaths of oceans (specially in the middle of Indian Ocean) in order to save power. They are not designed to track a fast moving target and have repeat cycle of at least a few days. Then there is the issue of resolution. High res acquisitions modes (with satellites like TerraSAR-X) are pre-programmed in advance.
So in theory it is possible that a flying aircraft might show up in the amplitude image (a pixel or two wide) if a satellite was indeed flying over the Indian ocean and acquiring data over the area where the plane was flying. But then all you have is a single image. And once the plane is under water you wont see anything.