According to Adrian Wojnarowski of Yahoo! Sports:
Y! Sources: The San Antonio Spurs hire ex-New York Knicks president and GM to front office. tinyurl.com/chxkdsv
— Adrian Wojnarowski (@WojYahooNBA) September 1, 2012
This will be updated if/when an official press release is given by the Spurs. A link to the article is in the above tweet.