I've read lengthy discussions of the archival properties of CD-ROM disks. But the glass masters from which commercial CDs are pressed might last millennia. The first generation of DVD readers will be backwards compatible, but I doubt CD technology in its present form will last as long as the phonograph. But all that kind of misses the point. One of the tasks performed by traditional librarians and archivists is to protect paper from the ravages of time, the elements, insects, fungus, fire, flood, and undergraduates.
Paper can last for hundreds of years, but only if it is taken care of.
Paper can also turn to dust in days or weeks. The analogous task for electronic librarians will be to protect their bits, independent of storage method or medium, by periodically transferring them all to whatever appears to be the most secure and accessible storage technology of the time, and by distributing copies of them to as many other widely dispersed locations as possible. But there does not yet seem to be an established tradition of digital librarianship to shoulder this responsibility and pass it on from one generation to the next. It's very difficult to establish traditions and a commitment to the long term when the technology is in such a state of flux.