Are you viewing this page with the latest version of whatever web browser you use? According to a paper quoted on Google's Security Blog, there's a good chance you aren't.
In the paper, researchers from ETH Zurich, Google, and IBM say they found only 59.1% of Internet users across the globe ran the "latest major version of their preferred web browser." The numbers paint Firefox users in a particularly good light, showing 92.2% of them surfing with Firefox 2 as of early June 2008, just before Firefox 3 came out. Internet Explorer users didn't fare quite as well: only 52.5% of them had made the jump to Internet Explorer 7 in that same time frame—and IE7 came out in late October 2006, a week before the launch of Firefox 2.
As Google summarizes, the paper points out that "technical measures now in place do not sufficiently guarantee browser security." The paper goes on to suggest browsers should make it clear to users whether they're running the latest, most secure release or not. For instance, "a visible warning [could] be instituted that warns the user of missing security patches in a way analogous to the 'best before' date in the perishable food industry."