I tried to access a Flash movie (OS 10.4.8, QuickTime 7.1.3, Flash 9). This is the movie;
http://www.youtube.com/watch?v=o-KWYYIY4jQ
Safari (v2.0.4) displayed a QT symbol with a question mark.
Firefox 2.0.0.2 displayed the movie without any problems.
The Flash option was enabled in QuickTime Preferences.
Following error messages from several sites, I upgraded to QuickTime 7.1.5 and Flash 9,0,28,0. There was no change; Firefox worked, Safari didn't.
After unchecking the 'Enable Flash' checkbox as recommended by Colin (thanks for that tip), Safari displayed Flash movies correctly. Firefox works as before.
All the web sites mentioned by JollyJim display correctly.
Ian