The City section of today's Times has a little blurb about underground cell phone service. For at least the last three years, I have always had 5-bar service (on AT&T Wireless) on the 1/9 inside Columbus Circle/59th Street station. This is not only because of the numerous vents, but also because the cell cluster is perched atop the New York Institute of Technology on 61st and Broadway, right nearby.

I think the emphasis should be on stations, not subways. People talking on their cell phones in the subway would be annoying. But in stations, it would be useful (such as when you are a 45 minutes late for brunch because of flooding rains). Montreal provides a good model. Cell repeaters were installed at major downtown Metro stops along the green line (McGill and adjacent stations), so you have service while you are waiting for the train. This is a good way to start and allows people to see if expansion is worth the cost.