AT&T's Synaptic Hosting Service [IaaS]
"Building publicly accessible cloud infrastructure is not inexpensive or uncomplicated. The service providers already have those infrastructures in place – the data center assets, connectivity and billing."
--Pund-IT analyst Charles King

AT&T's Synaptic Hosting Service , built on a virtualized "pay-for-what-you-use" infrastructure, delivers network, computing and storage capabilities. AT&T provides a holistic service level agreement to accommodate performance level requirements, but only up to 99.9% or about 9 hours of unexpected annual downtime. They are listed as a leader in the upper right of Gartner's December 2010 Cloud Provider Magic Quadrant. The full report is available for free from competitor Savvis.
In 2006, AT&T's purchased USinternetworking, an application service provider with enterprise customers in more than 30 countries. USi technology's five "super Internet Data Centers" in the United States, Europe and Asia, became regional gateways for the AT&T cloud network. (source: Network World)
We are searching for more information on the number of clients using Synaptic Hosting. Will add it here when we find it.
For the curious, here is AT&T's Q2 2009 balance sheet. Cloud is a tiny part of their empire and not mentioned in their statement.