The crux of my arguement is that it's too much to do and constitutes a move. I base this on the ruling vs unnatural or rough terrain. You can not take a five foot step to go up an incline or rocky/uneven surface. Air is not a natural MOVEABLE surface, not for a human, therefore you can not five foot step it. Below, taken from the SRD, empasis mine:
I present falling is still movement, as the player is intending to move this distance.
As I stated above, I believe air to be difficult terrain, and therefore you can not 5 foot step into it.
While there is no listed speed for falling, the devs can't think of everything. While a weaker point of support, as I believe the ones I stated are much stronger and to the point, it is still valid.
In response to the posts regarding illusioned pit traps and the like, i find these arguements to be apples and oranges. Falling into a trap is an unintendend consequence of taking a free 5 foot step. You though you were moving onto solid ground, and the rest falls into the slipping-falling category. In this case, the character is intentionally using the five foot step to move into air and, thusly, fall. Based on how it is both written and intended, this is an abuse of the rule, IMO.
Thoughts? Comments?