They are for sale again: http://www.theory11.com/playingcards/sentinels.php
According to Jonathan Bayme, they now feel like the Monarchs so I guess that means Bicycle 325 stock with Air Cushion (Premium 909) Finish!
It doesnt say that they are plastic coated but I don't think it said that on their site for the first print either so I really hoping that they aren't plastic coated and that they actually last more that 5 minutes.
While I probably won't get any of them, if any deck needed a reprint it's the Sentinels. Lovely design but MAN do those handle like a sticky pile of poo. They are far and away the worst handling deck I own, and I'm including the clear plastic Bicycle's in that equation. Actually now that I think about, I might get one so I can actually use them.
MDCasino, most of the stuff coming out of USPCC these days is pretty great handling. You'll hear most about Aristocrat, Bicycle and Bee stocks and general it's either the Premium 909 that MagicAddictz mentioned or Magic finish these days (correct me if I'm wrong collector experts). Air cushion is the embossed texture that probably almost every deck you own has on it, very few decks come with smooth finish.
Well, it's been a few weeks since I last posted this...
...but I'm just presenting the short version.
Despite the marketing gimmicks, USPC has exactly two finishes, standard and Magic. There's a third one being tested on a portion of the Vortex decks, but it's not available to others until the test is successful. The term "plastic coating" or "plastic finish" could easily apply to ANY of the varnishes used to finish a deck of playing cards - they're all some form of plastic, and have "plastic" qualities to them (see, it's not just a noun, it's an adjective)! "Premium 909 Finish" is a marketing term, no different than Ivory, Cambric, Linoid, Air Cushion, Smooth, etc. - it's the same standard finish that most other decks use, period.
"Smooth" and "Air Cushion" simply refer to the PAPER, not the finish. USPC has four stocks: Bicycle, Aristocrat, Bee and Bee Casino. Bee stocks are considered premium when it comes to ordering it for decks, while a basic order gets to choose between Bicycle and Aristocrat. Tally Ho stock and Bicycle Casino stock are no longer available. Each stock can be ordered with a dimpled (Air Cushion, Cambric, Linoid, etc. - it's all the same) texture or a smooth (Smooth, Ivory, etc.) texture on the surfaces. You might even try getting fancy and getting a stock like Jerry's that was smooth on one side, textured on the other, but I'd be afraid to ask how much they'd charge you for it.
Variations of these stocks can be created by storing the cut deck sheets under certain temperature and humidity conditions - the Aristo Banknote reprints and Tally Hos don't feel the same in how they handle, but they're both now made with Aristo stock, but they aren't the same variant, since the Banknotes are quite soft while Tallys are more stiff.
I'd assume that they used Bicycle stock and standard finish on the original Sentinels, but that the amount of black ink printed into the backs may have impacted the handling. It could even have been simply some inconsistencies in the paper company's manufacture of the paper before it even arrived in USPC's hands - you'd be shocked at how much the quality of a single type of stock can vary, and the more recycled paper that's used in it, the lower the quality of card stock it will produce. ALL stock papers are increasing their recycled content, so it's inevitably going to reduce quality over time unless someone devises a better way to make the paper.
Perhaps this time around it will be Aristocrat stock, since they love using it on other decks and it doesn't cost a penny more. I personally prefer Aristocrat to Bicycle. But don't buy into the marketspeak - there are nowhere near the number of finishes and stock most people are led to believe.
EDIT: so much for the SHORT version...