I happen to believe that cell phones have a very low probability of interfering with aircraft equipment. However, if there's even a 0.001% chance of an adverse effect, that means there could be some adverse effect roughly every three days (using the number I heard of 40,000 commercial flights per normal day of fight operations in the U.S.), statistically speaking.

That fact that several cell phones happened to have worked apparently without any adverse effects on the hijacked planes' equipment says virtually nothing about what the probability of a cell phone having an adverse effect on aircraft equipment.