I dug some information from a book called "Weather" by Mel goldstein.

"In 1963, when Hurricane Beulah was seeded, surface pressure in the eye began to rise and the highst winds were no longer concentrated at the center. In 1969, Hurricane Debbie diminished in intenisty after seeding, but the question always remained whether these changes would have taken place without the seeding. Also the number of hurricanes that have been available for seeding have been relatively few. By 1990, because of mixed, Project Stormfury was abandoned."

I know of some wineries (sp) in Germany (very dangerous hail over there) that employ the seeding method by plane or SAM's(that's pretty cool

). So maybe it does work, i don't know!
Keep in mind it's also an issue of politics and religious doctorines that I am not ready to get involved in. Basically it would be a good science fair project if your dad was the president.