Just curious...how many of these air-speed sensors are on board? I would assume there are several of them, so if one fails or malfunctions the others can still work.

Additionally, can't the pilots double-check the speed via GPS?

First off, the analog instruments, at least the airspeed indicator, still depend on the pitot tubes.  If the pitot tube is iced over, neither the analog nor digital airspeed measurements will be useful.  Second, in general you are correct about GPS indicating "groundspeed" instead of "airspeed", but. . . it would get you close to a proper reading (plus or minus the local wind velocity).  Iced over pitot tubes, depending on the amount of ice can give you grossly wrong readings.  This later case might be of interest if the readings throw the control laws in the flight control computers "off the scale".  For example, if a measured airspeed is impossible to achieve at a given altitude, there may not be any control laws in the computer that are applicable.  If you get into this kind of a "singularity" it is at least conceivable that a flight control computer might "burp" and produce totally non-sensical results.  In the Airbus, where there is no mechanical backup to override a faulty flight control computer command, who knows where the control surfaces might go and if the pilot inputs could restore normal flight afterwards in the case of such a singularity.  Im sure that there has been considerable testing on hot benches and flight simulators for these "out of envelope" conditions (and I strongly suspect that given the events here, there is much more going on now!), but. . . there are so many combinations that it would be totally impractical and actually impossible to test all of them.

The other thing on this topic that must be considered is the fact that while the flight control system certainly has got to be properly protected against the effects of lightning strike, what if some portion of that protection system failed?  For example the wire mesh sheathing in composite aircraft panels provides a "Faraday cage" effect to protect the interior of the aircraft (and the panels themselves) from the nasty effects of a lightning strike.  What if the mesh somehow failed to be properly grounded or was perhaps missing in one panel that was struck by lightning?  (Not suggeting this as the cause, just as one of the host of possibilities that might have to be considered if nothing else turned up.) 

If everything worked RIGHT, lightning most certainly should not have destroyed the Air France jet, but history abounds with cases of things not working quite right for a multitude of reasons.  In an aircraft solely dependent on a fly by wire flight control system, anything is possible if the lightning protection system did not work properly.

The flight data recorders would be VERY useful if anyone is ever to know exactly what happened.  Lets hope that they are found and are usable.

A closing thought. . . .When involved in an aircaft accident investigation, one of the FIRST rules of engagement is never to eliminate any options as to the underlying cause until the KNOWN facts force you to do so.

I doubt that only speed sensors could be lethal to such a successful model.. theres gotta be other reasons too !!
For example, has anyone taken into account that EMP from the lightning could have fried some of the other instruments.. its a more likely scenario coz directly, lightning has little impact on the airframe.