Flash works the same for me under 4.0b1 64-bit as under the stock Ubuntu 3.6 build on Ubuntu 10.04 x86_64.

Note that Flash doesn’t recognize mouse clicks properly unless this hack for nspluginwrapper is in place — *exact* same problem in 4.0b1 as under 3.6 — http://helpforlinux.blogspot.com/2009/11/i-cannot-click-on-flash-in-ubuntu.html

I’ve got nspluginwrapper and such already installed since ages, and it all seems to work just fine with, say, silly Flash games: http://games.adultswim.com/robot-unicorn-attack-twitchy-online-game.html

Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:2.0b1) Gecko/20100630 Firefox/4.0b1

Flash 10,1,53,64