PC makes a save...
The creature is helpless.
The creature can’t take actions.
The creature takes a -5 penalty to all defenses.
The creature is unaware of its surroundings.
The creature falls prone, if possible.
The creature can’t flank. -Rules Compendium p. 234.
However, saving throws are not actions. You still make saves against ongoing damage and death saves while unconscious.
Forced Movement and hindering terrain:
Hindering Terrain: Forced movement can force targets into hindering terrain. Targets forced into hindering terrain receive a saving throw immediately before entering the unsafe square they are forced into. Success leaves the target prone at the edge of the square before entering the unsafe square. - DMG p. 44.
There's nothing I've been able to find anywhere that precludes a PC from making a save throw vs. falling off the edge of a cliff for any reason.
While this breaks verisimilitude, it does so in favor of balance.