We weren't sure how easy this one would be to get to the bottom of, since hotel-casinos have a tendency to be secretive and paranoid about the strangest things. Happily, it turns out that bedding isn't one of them. Here's the skinny.
The mattresses in the rooms at Mandalay Bay are Serta Presidential Pillow Top, which are designed expressly for the hotel industry and feature Serta's patented Advanced Comfort Quilt for better circulation, which prevents tossing and turning. We found the queen-size version's available from Sam's Club for $1,348, inclusive of shipping. The comforter's a King Topper from Universal Hotel Supply ($64.30).
Bedding-wise, the sheets are Sobelcale 200 thread count 60/40 poly-cotton and are supplied by a company called Sobel Westex (http://www.sobelwestex.com/). They retail on their site for $119 per dozen, but it looks like it's industry-only and you have to sign in to make a purchase. The pillows are feather and are from Universal Hotel Supply, like the comforter.
As far as THEhotel's concerned (MBay's non-gaming sister tower, which has more of an urban theme), the mattresses are Spring Air brand's VIP Legend Pillow Top. The down crush feather comforter is by a New York company called Portico Bed & Bath (212/366-1163), which also supplied the pillows (outer shell is 30% down and 70% feather, inner shell's 95% feather and 5% down). The sheets are 250-thread-count Dolce Notte from Sobel Westex (as above) -- $322.50 per dozen.
So there you are -- and sweet dreams!