Only Flex apps that are started from Flash Builder are trusted automatically. So close all browser windows, check the list of running programs to ensure that the browser is shut down and then rerun your Flex app.
I am running it from Flash Builder. I can run Flex 3.4 applications fine, just not a Flex 4 app.
Quitting the browser did not make any difference. Thanks for the suggestion, though.
I re-installed Flash Builder 4 and immediately created the same application. I ran it from Flash Builder and got the same result. I am running on Mac OS X (Leopard). Perhaps Flex 4 apps are not runnable on Mac OS X yet? Or there is a problem running them from Flash Builder 4 Beta 2 on Mac OS X?
Probably time to manually check the trust files on your computer. I think I recall one other mac person who had trouble with trust files. FB would write them, but for some reason the player didn't see them.
Flex SDK Developer
Adobe Systems Inc.
The trust directory: /Library/Application Support/Macromedia/FlashPlayerTrust directory was empty so I created a FlashBuilder.cfg file in it and put into it, the path to the workspace where the files are stored. This allowed me to run the application.
Thanks for the tip; I had forgotten about that true store.