Ameritech, MFSDatanet and Pacbell have that information on-line:
http://www.ameritech.com/data/nap/customers.html
http://www.mfsdatanet.com:80/MAE/east.map.html
http://www.pacbell.com/products/NAP/nap2-5.html
The RA offers information on which ISPs are peering with
the RA service at those locations and the Sprint NAP:
http://www.ra.net/routing.arbiter/statistics/bgp.peers
--Elise