And the answer is:
It depends on the model of MacBook you have.
If you have any MacBook aluminum uni-body then the connector for the back lit keyboard is present on the logic board. The only way you can get the keyboard, without buying it directly from Apple, is to find someone who's laptop has had logic board problems, mostly due to water damage. Then you could replace your top case, with regular keyboard, with one that has the back lit keyboard and it would work as expected.
If you were still under a warranty from Apple then doing so would violate it. Though, from my experience damage done to Apple laptops is never covered under warranty because of obvious user damage or water damage. Your mileage may vary.
I am in the same predicament as you. I have the a model like mentioned above, meaning my logic board could power the back lit keyboard. But to me, any way you go it is not worth what it would cost/risk to do such an upgrade. I found a $5 solution, for when I am typing in the dark, and that solution is a $5 USB flexible keyboard lite. It's not sexy like the one the Pro has but it works. They sell them at the check out isle at Fry's Electronics.
I knew someone that sent in a MacBook aluminum uni-body, not covered under warranty (water damage), to a 3rd party repair shop. As a free bonus she got her repaired MacBook back after it had its top case replaced with one that has the back lit keyboard. But, I can't recall the name and doubt they are still doing that. Sorry, try Google.
OP: I hope that answers your question.
Encryption
p.s. In case you didn't notice: "Water Damage MY APPLE!"