Show me functional programming, I have no clue (obviously)

Functional programming is heating up. In several discussions over the blogosphere and on my blog others think I have no clue what functional programming is. After some consideration, I think they are right. I have no clue.

Well I do know what functional programming is. I've used it myself. But I have no clue how to do a business application in FP. Others don't know either. They talk about Bezier curves, square roots, fix points, y-combinators, power functions or remodel FP into OO and think that's still FP. Weird.

Suppose I need to write an application. There are customers who can buy products. Based on the customer and the products he bought he can buy other products. Those products he books do cost him money which he needs to pay to the company. I'm quite sure how to design such an application in OO, discuss it with some domain experts and write the code. I also know how to write such an application in a way that other developers in 5 years from now can read, undestand and extend the code.

But how would I write such an application in Lisp? Or Arc? Teach me.