And the answer is: a Zork

Don Sebastiani and Son's wrote a nice response to my post/e-mail below. The thing (the plastic, not-cork) is called a Zork, and though you would never know it from the Zork site, the winery was quite honest about the problems they'd had with the first couple iterations (that bottle we bought was from the first vintage that used the zork). (Mind you this is not a synthetic cork. Those have been around for years and seem to work just fine. This is, well, something else.) Maybe they've got it all worked out now, but all the same, I think I'll give them a few more years. It's like a new model car. Some of the best advice I've heard on that point was that the third year was the right time to buy--all the kinks had been worked out by then. Judging by what the DS&S had to say, I'm thinking it's about the same for new sealing methods in the wine industry.