Even a great technological innovation like PageRank will eventually be supplanted. For a web information retreival model, PR has already had a remarkable long life, yet its downfall has been predicted across the web by some of the very best in the industry. Here's a short list:

• Does Google still use PageRank Algorithm? - from SEW Forums
• New Versions of PageRank Employing Alternative Web Document Models - Research paper from Mike Thelwall & Liwen Vaughn
• How to Fix PageRank - from Threadwatch

As a global popularity model, PageRank also has some inherit flaws. PageRank is ignorant of the temporal nature of the web and links, favoring old content rather than updated or new information. PageRank also uses a flawed 'random walk' theory, which does not take into account the purposeful nature of a typical web session. Lastly, PR doesn't account in any way for the manipulation or commercial interest aspect of the web, and thus ignores the global popularity spam that prompted Google to show PR updates on a quarterly basis.