Ok, it's only an alpha release, but Adobe have FINALLY released a 64bit version of their flash plugin for Linux.
You can download it here (link is at the bottom of the page). Of course once you've got it you'll need to know how to install it, as Adobe just give you the library and leave you to work out what to do with it yourself...

But here's an idea of what you need to do to get it to work:

1. Uninstall any flash you may have on your system; for Ubuntu users just open a terminal and run:
   sudo aptitude remove flashplugin-nonfree
   
2. Go to where you downloaded the flashplayer to and extract it;
   tar xvzf libflashplayer-10.0.d20.7.linux-x86_64.so.tar.gz
   
3. Now you need to copy the plugin to Firefox's plugin directory; in Ubuntu Hardy Heron this is located at: /usr/lib/firefox-3.0.3/plugins/
   so you can copy it there with:
   sudo cp libflashplayer.so /usr/lib/firefox-3.0.3/plugins/.
   

Restart firefox and you've got yourself a native 64bit flash player!
enjoy.