If the grounds suddenly gives way, technically you shouldn't be allowed to cast feather fall since you haven't 'acted'. So you'd just fall and take a s%$@ load of damage. About the only way you could ever cast feather fall while falling, is after you've fallen the... what is it, 200 ft for the round you are surprised?
In combat, if you have acted, and then you suddenly fall, you could cast it. But outside of combat you are kind of just screwed.
This is why I think feather fall should have a 24 hour duration and it auto triggers if you fall at any time during the duration. It would be a sort of "baby contingency" spell and therefore require no special awareness or action on the casters part.
See also Emergency Force Sphere - immediate action, with text noting "Normally this spell is used to buy time for dealing with avalanches, floods, and rock-slides, though it is also handy in dealing with ambushes."
This seems to indicate specifically that you can use it flat footed. While immediate actions as a whole are not available when flat footed, there are a number of immediate action spells that seem designed specifically FOR use when flat footed.
Flat-Footed:A character who has not yet acted during a combat is flat-footed, unable to react normally to the situation. A flat-footed character loses his Dexterity bonus to AC (if any) and cannot make attacks of opportunity.
The upshot of this (and it's not obvious at first), is that combat must have started (meaning 'initiative must have been rolled') in order to be flat-footed!
If it's outside combat then you are not flat-footed! Read it again;-
'A character who has not yet acted during a combat is flat-footed.'
This means that you can take immediate actions at any time outside combat; the only time you can be flat-footed, RAW, is after initiative is counting down but before your first turn!