Is there really such a thing as an unlimited data plan?

They say everything old will eventually be new again, and as of this week, all four major wireless carriers are once again offering "unlimited" data plans to their subscribers. AT&T was the last carrier to jump on board, sort of — the carrier has actually offered unlimited data for a while now, but that plan had previously only been available to customers who also subscribed to AT&T's DirecTV satellite television service. Now it's available to everyone, which means anyone can now g