I thought this was pretty impressive. Kathryn Borel did this using what looks like a regular knife, but apparently a ‘champagne sword’ exists specifically for the purpose of sabring a champagne bottle. Other interesting trivia: Napoloeon was the pioneer of the art. Even more interesting is the physics of it (thanks Wikipedia):
A Champagne bottle holds a considerable amount of pressure. Early bottle designs used to explode and the manufacturers kept making them thicker until they could contain the pressure that is caused by the release of carbon dioxide during the fermentation. The inside pressure of a typical Champagne bottle will be around 90 pounds per square inch (620 kPa). The diameter of the opening is 0.7 inches (18 mm), so there is a force of about 35 pounds (16 kg) trying to push the cork out of the bottle.
At the opening of the bottle there is a lip that creates a stress concentration. On the vertical seam of the bottle the glass is not as uniform, which creates a second stress concentration. At the intersection of the seam and the lip, both stress concentrations combine and the stress will be more than half the strength of the glass. The impact of the saber on this weak point creates a crack that rapidly propagates through the glass fueled by the momentum of the saber and the pressure in the bottle. Once the crack has severed the top from the bottle, the pressure inside the bottle and the transferred momentum from the saber will send the top flying, typically for a distance of 5–10 metres (16–33 ft).
[Via Boing Boing]