When a blog checks for updates on plugins it sends as far as I know a list of all enabled plugins to the server to get information on whether a plugin is out of date or not.
I think it should be possible for the wordpress.org server to analyse those request and generate usage statistics for plugins from it.
Then we could have things like "This plugin is installed on X blogs".
This would be great for plugin developers to see know how much their "plugin" is actually used ("downloads" only gives a very rough idea about that because you never know if people actually install and use a plugin after downloading it).
With the data sent on
update-check it should also be possible to provide statistics for different plugin versions ("Version Y is still used on X blogs") and of course a timeline similiar to the "downloads" statistics one would be nice as well...