Been reading a few peoples suggestions for the forcefield and I thought I'd throw my own mishmash of stolen ideas together into one place.
My idea for the forcefield is completely different to the original design. Using a cable like device to create the shape of the field, much like dropping down buildcraft markers, only with the added ability of making complex shapes (well, as complex as minecraft can manage anyway).
The Screeny below shows the rough idea for the implementation. When the Forcefield 'cable' is laid down and then powered, it produces a field marked out by the black outline. As you can see, it would have to encompass the entire forcefield device as well to make it griefproof.
The black arrows in the middle indicate the checks that would be carried out by the 'ForceFence' to dictate whether or not it will turn on. If a corresponding fence block is in range and it is also powered, then a field will be produced. I would probably limit this to a certain amount to prevent massive fields that span the whole map, lol. Probably about 20-30 blocks or so? Or maybe smaller and require another 'ForceFence' to re-emit the field.
The field would cover the entire area shown inside the black lines. Think of it as replacing every block within, and including the fence blocks with a forcefield block (Only the texture would obviously be transparent and you could see your wiring beneath). This makes a forcefield box completely impenetrable.
For power consumption, I would probably work it something like a high resistance cable. i.e. it 'loses/costs' 5 eu/block but it will turn on the forcefield. I havn't done any math to support this number, but feel free to speculate. Based on the amount of cable required by this design, it may need to be a lot lower, compared to the old teleporter that is.