Yeah, there's a monthly article in Vintage Guitar magazine called 401K Guitars (or something like that). Over the past couple of years it has consistently provided data that shows investment-grade vintage guitars outperforming the stock market.

Then there are articles and comments being made about whether guitars are approaching a "market correction" like real estate has in the US the past year. As guitar buyers move to new generations, when will 59 Les Pauls no longer be attractive -- especially at $300,000? Guitars from the 70s have been appreciating despite the general belief that they are lower in quality...is that because the generation that grew up in the 70s is now buying the guitars of their youth? If that's the case, will interest in the guitars of the 50s and 60s wane? Or will people just say that paying more for a guitar than you did for your house -- or at least your car -- is no longer worth it?

Then there are other effects, like the fact that hip hop dominates popular music, and rarely uses guitars...

It will be interesting to see what happens the next 5 - 10 years.