There are quite a few security changes in the upcoming Flash Player 10, some of which may require changes to content or policy files to ensure that content continues to work.
Below are a couple of articles that discuss some of the security changes in Flash Player 10:
Understanding the security changes in Flash Player 10 beta
Working with policy file changes in Flash Player 9 and Flash Player 10 beta
Flash Player 10 FileReference Changes
If you developer or manage Flash content, then it is important that you read over these articles in order to understand the changes, and what, if any implications they have for your content.