Note: Do this with root access. If you don't know what root is, you shouldn't be messing with your N900
Replace the file in /usr/lib/browser/plugins with the file found here. Make sure that you already do a backup of the Flash 9 plugin and maybe move it to /opt to save some valuable space (note that the /usr resides in a paltry 256MB partition as opposed to your /home partition on the Nokia N900).
Also please take note that this plugin is BETA and is not hardware accelerated hence it will drain your battery rapidly.
Credits go to knownokia for this hack.