There's no evidence that a bomb didn't cause the disaster. There's also absolutely no evidence that it did.
Here's what we all know (in no particular order):
1) There was a bomb threat on another Air France plane earlier, but no bomb was found. This doesn't evidence anything to me...bomb threats are occasionally called in. The threatened flight wasn't AFR447 nor was it departing from Rio de Janeiro.
2) The plane wasn't in radar contact. This is perfectly normal. Radar has a limited range and there's no radar coverage over oceans. Position is verified by radio report.
3) The plane was an Airbus 330 with fly-by-wire controls. Some people have a problem with the degree to which the fly-by-wire system removes the pilot from the loop...especially in critical situations...but the fly-by-wire system's software will provide a better solution to a given problem than a human pilot's the vast majority of the time. However, if the fly-by wire's redundant systems all fail, it's practically impossible to control the aircraft. That said, the A330 is no more inherently "dangerous" than any other wide-body commercial aircraft. Some might disagree, but I believe that's mostly attributable to a "Boeing vs. Airbus" thing (think the "Chevy vs. Ford" argument).
4) Wreckage has been found. True, but it's a deep ocean and there's a great likelihood that the data recorders will never be recovered.
5) There was convective activity in the area. Pilots receive weather briefings before flights, but that's not even close to a surefire solution for avoiding turbulence. Neither is onboard weather radar, because it only sees precipitation...not turbulence.
6) Lightning poses very little danger to aircraft, especially commercial aircraft. The fly-by-wire system in the A330 is hardened against power fluctuations...as are all commercial aircraft, fly-by-wire or not. A given large commercial plane will be hit by lightning once every two years, on average. Smaller regional jets will be struck once or twice per year (because they generally fly at lower altitudes). The vast majority of these lightning strikes cause no damage whatsoever.
7) AFR447 left a large debris field. That could be attributed to a couple of things...either the plane came apart before it hit the water (bomb or airframe stress) or ocean currents and wind have spread the debris.
8) AFR447's ACARS system sent an automated message indicating electrical failure and a loss of cabin pressure. That suggests a failure of multiple systems.
What
some people know:
1) AFR447 was within the confines of the Intertropical Convergence Zone (ITCZ) when it crashed. The ITCZ is characterized by, among other things, particularly strong convective activity. (for more info on the ITCZ:
http://geography.about.com/library/weekly/aa050301a.htm )
Taking all of that into consideration, I think the most plausible explanation (with the data we have right now) is that AFR447 got a weather breifing and knew that convective activity was both known and forecast along its proposed route of flight. In the air, it saw high-intensity precipitation on its onboard weather radar and, as all flight crews will do, planned to alter course if necessary to avoid the worst of it (it's also possible that, based on reports, they planned to top it rather than fly around it). At some point, AFR447 encountered severe to extreme turbulence (explanation of "severe" and "extreme" here:
http://www.bayareapilot.com/turbulencereportingcriteriata.htm )
possibly combined with a lightning strike. The turbulence and/or lightning strike either caused both electrical system damage and structural damage resulting in a loss of cabin pressure
OR it caused structural damage sufficient to both depressurize the cabin
and disrupt electrical systems. The loss of cabin pressure suggests that the aircraft could have come apart before impact, but is not definitive proof.
That's it in a (rather verbose) nutshell. As a pilot, DemoTex will probably have more to add, but I think he'll agree with most of what I said.