> I must be missing something but I'm not sure what.
>
Here are the steps from my experience,
1) wget http://fpdownload.macromedia.com/get/flashplayer/current/install_flash_player_9_linux.tar.gz
2) tar xzvf install_flash_player_9_linux.tar.gz
3) cd install_flash_player_9_linux/
4) cp -v flashplayer.xpt libflashplayer.so /usr/lib/mozilla/plugins/
5) nspluginwrapper -i /usr/lib/mozilla/plugins/libflashplayer.so
It works well with my system (Lenny AMD64).
Please take note,
USE nspluginwrapper -i /usr/lib/mozilla/plugins/libflashplayer.so
INSTEAD OF nspluginwrapper -i libflashplayer.so