Do you mean like an Arc or Ellipse

I think JScriptChan is looking for something like NURBS surfaces. If you are looking for something like in this example (http://anar.ch/examples/CurvedRoof.htm), you might have a look at the construction. If you move the last slider (bottom), you see in RED the control points and in BLUE the resulting curves.

This explain a way to do it:

1-Create an array of curves (http://processing.org/reference/bezier_.html)
2-Get intermediary points on the curve (http://processing.org/reference/bezierPoint_.html), those points will become control points of curves in the other direction.
3-Create an array of curves in the second direction (you then have all the information required to create all your faces from this new set of curves)

I think there's many ways to do it, but you could think as if you are building curves on curves (in two directions UV).