As of September 2009, Flash Player 10 is installed on 93.5% of the personal computers in mature markets. This includes the US, Canada, UK, Germany, France, Japan, Australia, and New Zealand.
All the projects I work on target Flash Player 10 or higher, and I can’t keep count of how many times developers have asked me, “how can I make this code work with Flash Player 9”. Simple answer, you can’t, so stop asking ;)
Details are here:
Worldwide Ubiquity of Adobe Flash Player by Version