If you have Flash Player 10 Beta installed on your Mac, Mac OS X 10.5.3 will erase it.
This was done to ensure that everyone else got the latest security update, unfortunately the update package doesn’t run a version check to see if you have a Flash Player version newer than the one 10.5.3 includes. Which, right now, only wipes out the version 10 beta.
So, bottom line, as usual, update to 10.5.3 first… then go and grab Flash Player 10 beta again.