McDonald's Is Already Working This Into Their Value Menu

Found on Gizmodo:

The TasteScreen: a system consisting of an LCD monitor with a specialized USB device mounted on top of it. The device contains twenty plastic flavor cartridges which drip controlled quantities of flavoring into a staging area where they're mixed and dispensed.

When dispensed, the flavoring chemicals drip down to coat the monitor with a thin liquid residue. A user can sample the dispensed flavor by touching his tongue to the computer screen. Certain combinations of flavoring agents recreate a flavor appropriate to the user's task.