A
common frustration for me is the fact that wireless dead spots still
exist, even in 2009.  Fortunately for Chicago AT&T users that have
problems in the city's subway system, their suffering will end by the
end of this year.  Though they can't claim to be the first carrier to
the scene (fourth, after US Cellular, Verizon, and Cricket), AT&T
will offer cellular service in the Chicago subway tunnels.  In a
ten-year agreement with the Windy City, the carrier will install the
necessary equipment in the tunnels and will be live later in the year. 
The contract is expected to generate 3.1 million for the transit
authority.

Source: Cellular News