I just downloaded the Cliffhanger cards for the Savage World of Flash Gordon, and they present some interesting possibilities for improvisational gameplay. I'm still thinking these might present a challenge for the GM, but looking them over in detail, I can certainly see some possibilities for how to implement them in various games, not just in Flash.
Out of the Frying Pan: A much bigger problem appears, wiping out the earlier problem. This is perhaps a form of Summon Bigger Fish, but I could envision the bad guy in an encounter suddenly being squished by a much larger beastie...
Captured: This one is straightforward in description, but one of the harder ones to implement in practice. For whatever reason, the group is taken prisoner and must escape, fight in an arena, etc. I think a GM would require some seriously fancy footwork to capture the party arbitrarily in any randomly selected situation. Of course, there's always Rocks Fall, Everyone Dies...
Environmental Hazard: Another easy one - the ship catches on fire, the room starts to fill with noxious gas, quicksand out of nowhere. I see no problem dropping this into an encounter at all.
Disaster: This one is trickier - some sort of disaster occurs, dispersing the enemy and threatening the adventurers and the locals alike. Earthquake, sudden volcanic eruption, power plant explosion, etc. The reason I think this one is trickier is scale - you need a BIG disaster that fits the locale, and not every adventure takes place on a convienient volcano.
Perilous Portal: I like this one for getting a party that has gone off the rails back onto them. The party gets transported somewhere else, leaving the enemy behind. Finally, the GM can use the material he actually prepared for tonight!
Reinforcements: Another easy one - more bad guys show up, but the players get more bennies. This one looks like a lot of fun, especially if the players really take advantage of the extra bennies to wreak some havoc.
Overall, these cliffhangers seem workable in most situations, but I'd be a little hard pressed to deal with the Disaster and Captured scenarios without some prep. I think for the GM, knowing the players can invoke a cliffhanger at any time really requires some advance work and thinking to have some of these options on deck so that she's not caught flat footed.