As far as I know there is no roaming agreement other than the 911 calls between US Cellular and Verizon (or Sprint) in Chicago. This should not come as a surprise since Verizon is very heavy in Chicago, on a good note:
"A Verizon spokesman said the carrier is still evaluating expansion to the subway, and negotiations continue with the CTA. Sprint spokeswoman Stephanie Walsh said, "We have no plans to build our network out in the underground." "
(
http://www.findarticles.com/p/articl...5/ai_n16663700)
This article was written in 08/2006 so if VZW is going to allow service in the subway, I don't think it will be long now. I hope they do it, I hate not having service on the blue line, or the red.