Joel Fugazzotto offers some thoughts on his blog about what publishers and authors could do to make out-of-print and other legacy content available. This subject is also a subtopic within Lawrence Lessig's Free Culture, which I reviewed and recommended earlier.
Joel's point is valid. There's no reason for anyone (publisher/author) to sit on old content, especially since someone somewhere could probably benefit from access to it. In some respects, that's a problem Google no doubt hopes to eventually address with their Book Search service.
It becomes a thorny issue though as print-on-demand (POD) is becoming more and more popular. Publishers who previously would have been willing to let a book go out-of-print and revert rights to authors now see POD as a way to squeeze every last bit of revenue from the long tail. Individual titles may not cast off a lot of POD revenue but the income level can become significant when you have a very large list of titles contributing to it.
What to do? I think this has to be addressed on a case-by-case basis. As a publisher, if I feel I can still generate a reasonable amount of income off an old title via POD, and therefore it should also continue to generate some income for the author, I generally want to hang onto it and the associated rights. That said, I'm also sensitive to an author's needs and interests and have certainly reverted rights on plenty of books over the years.
If you're an author who has a "legacy content" situation with a publisher, my advice is to hook up with your editor/publisher and discuss the options. Don't just let the content sit. For all you know, they might not even have explored POD as a solution for your book yet!