From an article on Newsweek:

“You don’t want your phone to be an open platform,” meaning that anyone can write applications for it and potentially gum up the provider’s network, says Jobs. “You need it to work when you need it to work. Cingular doesn’t want to see their West Coast network go down because some application messed up.”

I’m not sure I really understand this remark. There are other programmable smartphones, and you can get GSM adapters for some PDAs as well. None of those seem to worry network operators.