For those running Ubuntu AMD64, you can put both 32 and 64-bit versions of libflashplayer.so in ~/.mozilla/plugins (call them libflashplayer32.so and libflashplayer64.so if you like), and 32-bit Firefox will load the 32-bit plugin, and Chromium 64-bit will load the 64-bit plugin. You can get the 64-bit Flash plugin here:

http://labs.adobe.com/downloads/flashplayer10.html

And if anyone's wondering why I'm not running 64-bit Firefox 3.5, it's because the Ubuntu (and possibly Debian, I've no idea) packagers still have it branded 'Shiretoko', which throws off every site searching for 'Firefox' in your UA. Hopefully by Karmic's release they'll have this sorted out.

Also, why does Mozilla not provide 64-bit builds?