Flattery will get you everywhere, so please find the complete list below. But first ...
Champagne is a sparkling wine named for the northeast region of France around the cities of Reims and Epernay, from which it originates. True Champagne only comes from this specific area.
For centuries, Champagne had been producing some of the most highly regarded still wines in France, used for the sacrament, coronations, and the royal table. Most of these wines were drunk when they were very young and without bubbles. The cold climate and short growing season of the region mean that the grapes are, of necessity, picked late in the year and this doesn't leave enough time for the yeasts present on the grape skins to convert all the sugar in the pressed grape juice into alcohol before the cold winter weather halts the fermentation process.
If the wine isn't drunk before spring, the onset of warmer weather kicks off the fermentation reaction again –- refermentation, as it's called –- producing carbon-dioxide bubbles. Originally, the wine was stored in non-airtight wooden casks, which meant any effervescence escaped, but once the switch was made to bottles, those bubbles became trapped. While today that sparkle is a big factor in what makes Champagne so prized, back then it was the bane of the winemakers, who spent a great deal of time trying to prevent the bubbles, not only deemed a sign of poor wine-making, but which would often result in bottle explosion.
The famous Benedictine monk Dom Pérignon, cellarmaster at the Abbey of Hautvillers until 1715, is often credited with inventing Champagne, but in fact his contribution was in the blending of Champagnes and the taming of the unstableness of this "mad wine," as he called it, by using thicker bottles and Spanish cork instead of the wood and oil-soaked hemp stoppers then in use.
Once tamed, Champagne's popularity spread worldwide as the drink of choice for special occasions, its high price dictated a) by the small growing region and b) the labor-intensity of the production process.
As far as France is concerned, there's only one region from which true Champagne emanates and it's regulated by law that only wines from this region can bear the name. It's administered by the Institut National des Appellations d'Origine and as well as the delimitation of the area from which a true Champagne can come (approximately 84,000 acres of northeastern France), the Appellation also carries 35 rules to uphold the quality of Champagne wines: Only 3 grape varieties (Pinot Noir, Chardonnay, and Pinot Meunier) are permitted; yields in both the vineyard and during pressing are limited; and the pruning of vines, their height, spacing and density, harvest by hand, and minimum aging periods are all controlled.
Other areas produce sparkling white wine, known as 'vin mousseux' or 'vin crémant' in France, cava in Spain, spumante in Italy, and Sekt in Germany. America has no such regulations as France, so U.S. laws do not preclude vineyards labeling domestic sparkling wines as Champagne, much to the chagrin of the French Champenois, as the official producers are known. They are often excellent and the latter may use grapes imported from Champagne in France and come at considerably cheaper prices than the real thing, but to the purist they are not true Champagnes.
Which brings us to the pedantic answer to your question, which is that only one Las Vegas casino champagne brunch actually serves a good and authentic Champagne, namely the Sterling Sunday Brunch in Bally's Steakhouse, which puts out the very respectable Perrier Jouët (and lashings of it). All those venues that offer J. Rogét are also serving an actual Champagne from Champagne, but it's about as low-end as you can get (we've seen it for sale online for as little as $1.99 a bottle). Quality-wise, Paris probably comes next closest to Bally's with their Saint Hilaire, a sparkling wine that claims to be France's oldest "vin mousseux" but comes from the south of the country, so can't be designated a Champagne. Most of the other brunches serve sparkling Californian wines, while Bellagio and Wynn serve Freixenet and MGM Grand serves Brut Blanc de Blanc, both Spanish cavas.
Here's the full list:
Note: *Brut -- denotes Driest **Extra Dry -- Medium Dry
And lastly, we'll leave you with a fabulous quote from Mme. Lily Bollinger, descendent of the founder of the famous Champagne house, who ran the business from 1941-1971 and took it upon herself to extol the virtues of her product vociferously:
"I drink it when I'm happy and when I'm sad. Sometimes I drink it when I'm alone. When I have company I consider it obligatory. I trifle with it if I'm not hungry and drink it when I am. Otherwise I never touch it -- unless I'm thirsty."
