A few comments on the article claim that it has been Apple's worst kept secret in recent memory .... I agree.
It was the first time that Apple had to coordinate a launch with foreign sources - international partners apparently had no sense of loyalty or commitment to integrity as Apple's American business partners do. International carriers had no chance of losing the exclusive contracts for the iPhone - so their word to keep things tight lipped really meant nothing.
I think Apple's shift into a well established "type of market" like the cellphone business is indicative of how well a secret can be kept. Apple can threaten any Apple employee with termination* and in some cases they can threaten a supplier with contract breach; thereby ending a lucrative contract for them.
Apple couldn't threaten AT&T employees and essentially could not go with any other carrier in the United States for launch of the iPhone. So ... even if Apple had gotten mad at AT&T - it's not like they could have (or would have) gone with T-Mobile - the only other decent sized GSM carrier in the USA*.
One other contributing factor to the leak of "trade secrets" by AT&T employees is unionization.
A large number of AT&T Mobility employees are unionized, belonging to the Communication Workers of America. The CWA represented roughly 15,000 of the previous 20,000 formerly AT&T Wireless employees as of early 2006. As of the end of 2007, the CWA website claims roughly 40,000 workers of AT&T Mobility are represented by the union.
A union can can act as a safety net as an employee. Usually the only time you are fired (if unionized) is by full scale cutbacks. Just as a governemt employee is rarely fired - so works the union process at most large companies.
* T-Mobile subscriber base - estimated at 31 million and not established in all Top Markets in the country
* AT&T subscriber base - estimated at 74.5 million and established in 92% of the USA.
* Apple's employees for the most part take pride in keeping things tight lipped and employee handbooks talk about the benefits to their stock and to sales if they do. Higher sales do after all equal more chances for raises and bonuses.