There's usually a hotspot open in the Dining Concourse (lower level of the Terminal). Hours listed above apply to shops, not wifi. During business hours, there's lots of good (if slightly expensive) food, including Junior's Cheesecake. Homeless tend to congregate (read: sleep in the chairs) in the Concourse after 9pm, though, so watch your hardware. For more info on the Dining Concourse, check http://grandcentralterminal.com/pages/getpage.aspx?id=F7E5588F-54F7-495E-A6F6-87FFEF54306C