The 12 pool balls problem

Not my pic... click for originator

O.k, here's one for ya...

You have twelve pool balls and visually they're all exactly identical. One of them, however, has a different mass/weight to the other eleven, but you don't know whether it's heavier or lighter.

You have at your disposal a set of weighing scales so you can weigh the balls against each other, but no weights to go with it. You can only use these scales three times*.

Your job is to find out two things:

1. Which ball is the odd one out?
2. Is it heavier or lighter?

The rules

1. You're only allowed to use the instrumentation and apparatus as described above. No magicking weights or digital scales out of thin air, or inventing science prep rooms nearby.
2. No cheating. There are solutions available online, but if you look them up you're only cheating yourself. If you look it up and post it here, then you degrade yourself to a point a little beneath most traffic wardens.
3. There is a solution.
4. Feel free to comment and ask for clues. My HOD and I have worked out what we think is a pretty nifty solution, so I might post a clue-post in a few days or so if anybody asks for one.
5. I will post my solution at a future date as yet undecided, then edit this post to link to it. If you're reading this after that's happened, please give it a shot on your own (or with friends/colleagues) first. It's more fun that way!
6. Feel free to post your own solutions on your own blogs or websites and then let me know so I can link to them.
7. Subscribe to MathsQS for updates and new posts!

* No, I don't know why. Maybe they're set to explode, or something. Think Die Hard II with the jugs problem.