Remember when Adobe and ARM were working on Flash 10.1 for Cortex-A SoCs and they said devices would ship with Flash 10.1 in 2H 2010? Well, it's Q1 2011 and we've yet to see officially Flash 10.1 on the very device it was developed on - the N900. Luckily, they did give out the plugin on a case-by-case basis. Well, the internet is a vast and anonymous place, blah blah.
Here is the plugin on dropbox. To install it, simply replace the shared-object in /usr/lib/browser/plugins - but be sure to create a backup in the event that you need to revert. I suggest you move it out of /usr/lib and into /opt as it'll chew roughly 7MB of space. Restart browserd or reboot your device and you're done.