I can't believe Minneapolis hasn't been mentioned! Some love em, some hate em, but we do have one of the largest systems, the largest in some ways:
http://en.wikipedia.org/wiki/Skyway
The world's largest skyway network – Calgary, Alberta's "+15 Walkway" system – has a total length of 16 km (10 miles). However, the system is discontinuous, and it does not connect every downtown building. The largest continuous network of skyways – the Minneapolis Skyway System – spans 8 miles (13 km) connecting 69 blocks in downtown Minneapolis.
Of course there are many arguments on how much and in what ways they effect street life, and how streetlive can be enhanced and cohabitate with the skyways, but thats another debate entirely.
http://2.bp.blogspot.com/_T-wFiWf9O2...2005-09-27.jpg