That's a very interesting question. Theoretically, any open spec should be readable forever, but in practice that's not really the case.

That issue, by the way, is why I never recommend that someone shoot in RAW mode on a digital camera. Aside from the massive inconvenience of having to "develop" the RAWs by hand, for very little increase in quality, the formats are also all proprietary, and have no lasting power. The JPEG spec isn't going to be forgotten any time soon.