If you bought Flash CS4 already, you might want to check out the Flash CS4 updater for Flash Player 10. You can read about it, and download the installer, here:
Adobe Flash Player 10 Update for Flash CS4 Professional
What is it for? I quote “The update replaces the Debug and Release versions of Flash Player 10 browser plugins, standalone players, and Test Movie players that are included in the initial release of Flash CS4 Professional (player version 10.0.2.54). All users are encouraged to apply this update. These new players are version 10.0.12.36.”
UPDATE: Please note that the original release notes for this updater noted test movie players, however test movie players are not updated with this update. See comments below.