How can the current viewport view be rendered as a preview?
Rendering the Current View
The easiest way to do this is this:
You can read more about rendering in Blender here on the Blender Wiki.
Quickly Previewing the Current View
If you don't need to save the image file but you want a very fast way of previewing the render, position the viewport how you like it and hit ShiftZ. Hit the key combination again to leave the view.