Right now, Verizon is charging about $30 for unlimited smartphone data, the same as AT&T's plan prior to its change to tiered pricing; AT&T is now charging $25 for 2 GB of data and a paltry 200 MB for $15.
The move is justified by the carriers in that most users use under 2 GB of data a month; for a majority of cell phone owners, it will be a cost savings of $5 though users on AT&T's plan under the unlimited data terms are grandfathered in unless they explicitly call in and change to a tiered plan. For heavy data users--a minority--this will help to offset network congestion and hopefully provide a better experience for all users.
However, despite those justifications, as carriers move to 4G networks--AT&T and Verizon Wireless are opting for LTE technologies--that are more efficient at handling data, charging more for data flow is controversial as it'd be cheaper for networks to handle data under 4G systems. Another reason for controversy is that as smartphones get more powerful, data caps may limit the potential of your "pocket computers." For devices capable of shooting large megapixels worth of pictures and high-resolution HD videos, data caps may make it prohibitive to share and be social on the web while mobile.
At this point, such a move seems likely though Verizon Wireless has not officially commented on when a change would occur.