I've watched that closeup video of the plane hitting the tower with the "two explosions" and I can understand where one could see the first explosion as before impact, but I see it differently. Just before the plain impacts the WTC, the tip goes into the shadow of the building. That's why there appears to be an explosion "before" the impact. The plane extends forward beyond what is being lit by the sun to the point of impact, where an explosion is pretty likely.
This conspiracy theory, however, pales in comparison to the horrific rumors spread in the Arab world that blame 9/11 on Israeli covert operatives. I was deeply hurt when my close friend explained this theory to me as if it had credibility. She's not stupid--she's been educated in a high-quality public school, her mother has a good job, and she did quite well in her AP classes in high school. That was the scary part. If she, an American-educated moderate Arab, put stock in this rumor, how widespread must it be in Arab and Muslim areas of the world? Ghostface, can you confirm the existence and proliferation of this rumor?