Interesting....
There was a discussion about the different revisions somewhere, gonna find it.
Purely from memory... I believe that the info was:
All DS models with v5 or later firmware have backlit screens.
Many DS models with v4 firmware have a backlit screen, with unconfirmed exceptions.
Several v3 models (Depending on region) had a backlight, mainly the special edition ones, such as the mario kart DS.
V2 is frontlight
V1 is frontlight
Ique DS (The Chinese DS) has backlight starting from firmware 1 models.
Japanese models v3,4,5 all had a backlight, no other info on earlier generations.
Thanks for the info Orkie, I will make a note of it.