Just what the title says; In a closed environment like a subway tunnel, or a deeper subterranean tunnel - will a GPS enabled phone be able to provide lat/long?
GPS relies on your device picking up radio signals transmitted by satellites orbiting above us, an awful lot of things can block these relatively weak signals. From Pocket GPS World: How does the Global Positioning System work?
This is the same whether you're using a dedicated GPS receiver, or a smart phone, if the radio signal can't get through your device won't be able to pick it up.
In addition phones tend to have less sensitive GPS receivers than large, powerful, dedicated GPS devices with large antennae so there are likely to be marginal places undercover where your phone won't be able to pick up a GPS signal when a dedicated device may be able to.