logic puzzle

I think its about time I made a much less serious and more light hearted post so here goes.

This is a really good puzzle one of my friends gave me a couple of years ago, be warned its not for the faint hearted.(expect to spend at least a couple of hours racking your brains before you get a working solution)

You have 12 identical looking weights, 11 are the same weight one is slightly different (you can't tell the difference by hand), there is also an old style balance scales (think the star sign Libra). Using only three weighings devise a system where you are guaranteed to find which weight is different and if it is lighter or heavier.

Also all weights being tested at any given time must be places simultaneously so no cheating by putting one on each side then adding pairs until it goes off balance and calling it one weighing.