It seems that Confluence 2.2 automatically/silently disables plugins when they are uninstalled, and subsequently reinstalling the plugin (or upgrading it) doesn't re-enable it.

I came across this while upgrading a plugin via the Plugin Repository client, which (1) uninstalled the old version and (2) installed the new one, but didn't re-enable it. As a result the corresponding macros were unusable until I had a look at the regular admin/Plugins page and saw all the red stuff. Simply clicking on Enable plugin solved the problem.

The adaptavist guys say they will fix the repository client so that it re-enables plugins after upgrading, but is this a feature or a bug from Confluence 2.2?