Prufrock (if you're not subscribed, you're ill-informed) linked to a piece in the New York Times last week about vast quantities of fine that are locked away in storage in free ports around the world. (If you want to know more about that or what a "free port" is, I commend the article to you.) The article notes that a number of works sold for more than $100M are among those stored away. Which is a bit ridiculous.
So, yeah, okay. In one sense they're worth every penny someone is willing to pay. But the possibility of a bubble can't be ignored. Can it? Art is worth millions because people will pay millions until it isn't worth millions because no one will pay millions. With the risk of forgery, the prices climbing steeply, and the potential for changes in fashion and fad, isn't the art world ripe for having the bubble burst?
Maybe I'm wrong and baseball cards are going strong, but they seem like an instructive parallel. In the 80s and early 90s baseball cards were all the rage. Books were published (oh, the days of yore! before these intertubes) detailing and cataloguing their various incarnations, rarity, value, and oddities. There was a strong sense that complete sets, rookie cards, autographs and the like represented investments with a long-term future of continued appreciation.
The the market glutted, the fad passed, and the bottom fell out. It's not a perfect parallel, of course. There is no way to legitimately increase the number of Renoirs or what-have-you floating around out there in the world. Old Masters were prized long before baseball was invented, much less baseball cards and great art has more intrinsic worth than even the rarest baseball card. But the shift to art being seen as an investment rather than valued for its own sake seems eerily familiar.