Tactus

A morphing tactile display for touchscreen devices:

[The] multi-layered Tactus display technology sits on top of a device's touch sensors and has "micro channels" comprised of multiple tiny holes in predetermined locations; when physical buttons are needed, an optically clear fluid rises up from a lower layer of the Tactus sheet to fill the holes and create the on-demand physical interface. When physical buttons aren't needed, the fluid exits and the display flattens back out

There's a vid at the link.