<< why is everyone so sure it was bin laden? (not that i'm a terrorist or anything), just casting my doubt because theres no proof yet. >>
Direct evidence connecting Osama to the attacks don't yet exist, and might not exist for months or even years.
However, he is at the top of the suspect list for two reasons. First, attacks of this magnatude suggest a group with very large operational capabilities. Second, the group must have well developed infrastructure. Al-Qa'ida is the only known terrorist organization who meets both requirements.
In addition, Al-Qa'ida's connections and coordination with other militant groups gives them access to commanders with years of experience in everything from suicide attacks to plane hijackings..