Hey =] I work at a costa, so just thought i'd explain for you.
What makes the flat white so different to say a cappucino or latte is the way the milk is textured. Its done in a way to create micro bubbles, which is what makes it so smooth.
This happens because of the reaction the enzymes in the whole milk has with the heat of the steam.
They can be done with skimmed, but when skimmed milk is treated to remove the fat, some or most of the enzymes are killed off. So its not as smooth. You get more air bubbles in it, and its harder to draw the florette =P
I always explain this to customers, but if they're ok with it, i am =]
Hope this helps x