Here is the main indicator that this is going to happen: PR has been removed from the Google Toolbar. This is what they do when they want to phase something out.
As a measure of the page importance, Google PR has never been very valuable. While Google internally maintains the floating number for the PR which is updated daily, they only update it publicly 3-4 times per year. When they do, you get an integer, not a floating number, so there's no telling whether PR 5 is really PR 4.51 or PR 5.49.
The main reason for getting read of PR is simply the fact that ordinary people hardly know what it is, let alone use it - it's a measure used almost exclusively by marketers.
In short: I could be wrong, but I think that some time in 2013 Google is going to discontinue with the Pare Rank.