Ohhh... right
Feather Fall
The addition of immediate actions to the game means that casting feather fall is now an immediate action instead of a free action, since you can cast the spell at any time, even if it’s not your turn.
So Hypertext has been updating stuff from the RC??? nice... didn't know that.
As for your question... I don't find it problematic. You remain flat footed until the moment you slip...
from the slip onward you can activate it... makes sense doesn't it? Or perhaps you are asking something else?
This is how I handle it.
There was lengthy discussion on this very topic as it pertained to falling traps.
My opinion (and is pretty much how I see the rules as written) is that all of a character's movement is resolved on his turn in the initiative order. That means the character is no longer flat-footed when you start to resolve the fall - hence you can now take immediate actions and feather fall works as intended to. There is a specific exception to the resolve movement on your turn rule and that would be bull-rush but that is specifically stated when to resolve the movement hence it is a situational/specific exception to the general rule.
So while it looks Whiley Cayote like a character (per the rules) is momentarily suspended in air for the period of time between the action that caused him to fall and when he actually starts to fall. This is a peculiarity in the rules, but since the combat rules are be design very loose in the real world logic/physics comparison it still fits within the rules as written.