They get a weakened down version of fly, which allows them to get to areas that would need to fly. It would cost 50 mana initially to use it, reason being that it takes concentration to do so. Also it would take an additional 10 mana per movement square, and the movement would have lag on it, 1 tick. If the Drow is attacked during this time period, they no longer fly. They also cannot levitate if their adrenaline is still gushing.

I personally hadn't ever thought of the dark-elf or drow, however we are looking at it race, to have an inherent interest or quality to actually levitate?
They spent their lives in the underdark, underground.
I really do not see why they'd ever need, either historically or otherwise of a natural levitation?

In traditional drow legend, they can faerie fire people and levitiate themselves.

Drizzt does it in the story where he goes back to the underdark. (starless night???).

Living underground does not mean levitation would be inappropriate.

For example, a drow might want to lurk in ambush amongst staligtites.

I gotcha, yea I was not sure myself, having no time to read, yet always had the ability to mud on occasion. :)

At the same time, it has been said that the Immortals intend a gradual shift of the dark elves away from D&D drow. Or at least gradually away from the Forgotten Realms variety famous to the Drizzt books. (they did originate in the Greyhawk setting first, didn't they?)

Why? The Forgotten realm Drow rock!