There is a Flash 10 64bit alpha. It has been getting great reviews and seems to be more stable than the 32bit version in Ubuntu.

Download the plugin here

These instructions are taken from adobe's site:

Installation

Linux

1. Download the plugin to begin installation. A dialog box will appear asking you where to save the file.
2. Save the .tar.gz file to your desktop and wait for the file to download completely.
3. Quit your browser.
4. Remove all existing Adobe Flash Player installations from the system.
5. Unpackage the file. A directory with contains libflashplayer.so will be created.
6. Copy libflashplayer.so to ~/.mozilla/plugins. Create the 'plugins' folder if it does not exist yet.
7. Launch your brower. To verify installation in Firefox choose Help > About Plug-ins from the browser menu.


Reference http://labs.adobe.com/technologies/f...tes_64bit.html