is it possible to have displacement from a displacement map visible in the viewport, or is it only avaliable post render?
You get it with Shift-P
that shall have to do. I was hoping that it could displace the mesh in the viewport and not have to render everytime 
No, you don’t need to render. The preview window is just a window you put over the place you want to see prerendered. Any change you do to the scene is updated inside the window (a sort of transparent window inside wich everything is seen as if it was rendered).
The problem is it can be very slow with complex scenes.
hmm, it wont come up when I press shift P. Infact nothing happens at all. it also doesn’t saying anything about it in the hotkey reference part either :S
Are you working with a recent CVS version? Not standard 2.41, but something like the Orange Edition included with the Elephants Dream DVD, or a pre-2.42.
An alternative is to load your greyscale height map into a texture channel, go into edit mode, select the editing(F9) panel in the buttons window, and under Mesh Tools start clicking the Noise button, you will see your plane start to take shape, keep going until your mesh is the scale you want.