The intel Macs (iMac, MacBook Pro) ship with a custom compiled version of the Flash Player plugin. If you go to “upgrade” flash from Macromedia’s site, your Flash plugin will no longer work.

There are two solutions:

1. Go to the application, control+click on the application, select “Get Info” and check the box that says “Run using Rosetta”. This box is only available for apps compiled as Universal binaries. Applications that are compiled only for Intel or only for PowerPC with applied environments automatically. (Note that when you run an application under Rosetta, you are taking a speed hit. That’s why I wasn’t happy with this solution.)
2. This option is to insert your Install OS X Disc 1 (do not open the disc), download Pacifist and click “OS X install Packages” then browse to Essential.pkg\Library\Internet Plug-ins and extract “Flash Player.plugin” to \Library\Internet Plug-ins on your harddrive. This restores the Intel compiled Flashplayer.