mIRC (With VRockers DLL) :
on *:SIGNAL:vcmp.player.move:{
  ; $1 = ServerID, $2 = PlayerID, $3 = OldX, $4 = OldY, $5 = OldZ, $6 = NewX, $7 = NewY, $8 = NewZ
if  ($5 > $8) { 
if ($calc($5 - $8) > 1) vcmp.msg $1 $2 You're fallin' motha f*cka. 
}
}
  !.signal vcmp.move $1-
}  
Pawno: 
Text version :p Make a timer that detects Z movement of every player at every 500 milliseconds or so, it's best to do this in a DLL for max speed. Make it store data into a 2d array. First dimention for player's id and the second one is for the old/new cord. Once the timer is done, use the same logic as I used in mIRC. Simply compare the older Z location with the newer one. That's pretty much it.