1) contours of the stream function delineate flow paths, so a boundary with constant value of the stream function will be an impenetrable wall. A Natural(.)=0 will cause the contours to meet the wall at right angles, so this BC describes a penetrable opening.
2) from the definition of the stream function, you can see that a coordinate-dependent Value BC will specify the velocity flowing through the boundary, e.g. Psi=y means a uniform velocity crossing a vertical wall.
3) Boundary conditions for the vorticity are more difficult to define, but there are some notes on this issue in our version 6 example "Samples|Applications|Fluids|Buoyant.pde" ("Samples|Steady_state|fluids|buoyant.pde" in version 5).