On pinger life,
this website says that
Dukane, the locator beacon's battery manufacturer reports battery life drops of significantly after 30 days.
Dukane, the AF447 pinger manufacturer, has confirmed that the pingers may only last a day or so longer than the specified thirty days. Unlike regular flashlight batteries that fade out slowly, the battery technology used in the pingers will hold its voltage for the thirty days, and then quickly collapse along with the transmitted signal.
On underwater search methods,
the same site explains how an untethered Automated Underwater Vehicle (AUV) was evaluated to search for AF447's locator beacon.
Like the unmanned drone aircraft used by the military, AUVs are unmanned, untethered, computer controlled underwater vehicles. C & C's 4,500 meter rated vehicle is capable of searching large areas while flying at a constant height off the ocean bottom at four knots for two days at a time before returning to the surface to refuel.
Here is
an interesting whitepaper, discussing how the Ark Royal was found, etc. (note: link downloads a .pdf file).