Apple updated Safari recently to 5.1.7, which comes with a new security feature that checks the version of Adobe Flash Player installed. If it is or earlier, the Flash Player.plugin file is disable (moved to /Library/Internet Plug-Ins Disabled), and then Safari redirects you to the Flash Player Download Center to obtain a current copy. This is a one time check, and you can en-enable outdated versions of Flash if you have a need to do so, although running older versions of Flash is risky from a vulnerability standpoint.

Interestingly, Apple also released a Security Update 2012-003 for Leopard, the first security update for OS X 10.5 in quite a while. Its sole purpose is to disable versions of Adobe Flash Player that do not include the latest security updates, just like with Safari 5.1.7.