Not allowing certain products to have their price changed

Standard
So this question was asked today on the salesforce.com Answer Community.
I am trying to put together a validation  rule that will only allow two of our product codes to have a manually adjusted sales price.
I responded with a few things but unfortunately didn’t put as much thought into it as I should have. The person was having troubles getting it to work so I decided to dive in deeper and get this one for them. It ended up being much easier than I expected.
So the point of this validation rule is to make it so that only certain products can have their prices changed at the opportunity line item level. Certain products should always be the list price no matter what.