No one's exactly sure when AT&T Corp. got Internet religion. The idea that the Internet will be the driving force in 21st-century telecommunications has been percolating for a while, and as far as anyone can recall, there was no single moment at AT&T when the light bulb went on. But somehow the company has changed its entire mind-set -- identity, even -- from a tired long-distance phone company to one of the most bullish Internet communications forces in the world. The obsessive network acquisitions of CEO C. Michael Armstrong have attracted the spotlight, but the transformation seems to have happened before his arrival at the company. And it seems to have happened far away from the executive suite, in the enclave now known as AT&T Labs, where small groups of Internet researchers quietly mapped out the revolution to come.