let me drop my 2 cents...
it took a year and a half for the Flash 5 player to reach over 80% saturation. I remember when Flash 5 first came out, for about 4 or 5 months after, we were still producing for Flash 3! Simply because the penetration of even the flash 4 player was not high enough for our clients.
If you are going to be deploying your work on the web, make sure the client knows what's going on. Give them some stats on plugins and let them know what the options are. Make sure they're ok with putting up a screen that says "you must have Flash 6 to view this site." Or, if they want the vast majority of people to be able to view their site with no problems, make it compatible with the Flash 4 player.
There is no way for everyone in the world to simply update their players to version 6 overnight.