Note that plugins can also be installed "locally" to your user. In this case the .so file and friends go in
...where ~ denotes your user's home directory. When installing the flash 9 beta, the process is just downloading the .tar.gz file, untarring it, and manually copying the libflashplayer.so file to that directory. The down-side of this method is that it affects only one user at a time. If you have multiple user accounts on the machine, it's best to copy to the central location.
I think flash version 7 has some installer to decide where to install. personally I find these installers more confusing than manually copying the file, but I am told I'm odd, so it might just be me.