One thing to remember about designing an AD...
DON'T LET MANAGEMENT SEE THE DESIGN!!
Remember, AD is for the techs, and only the techs. It is really ment for rolling out GPO's. Starting from the least restrictive, down to the most restrictive (in the outer branches).
Once management takes a look at a good AD tree, they just say "That isn't how the company is laid out!! Change it!" Once you make the change to how the company is laid out, your life will be a living hell from that day on. Trying to update policies, permissions, etc. You will end up breaking something down the tree that you will regret.
So just keep in you mind of laying out OU's that they don't need to match the companies hierarchy. Think about the machines and what they do, it might overlap and match the companies layout (smile when that happens), but keep it as simple as you can.