I've just upgraded my test wiki to 2008-05-05, and noticed this problem when upgrading my plugins via the plugin manager:
I was able to click on the "update" buttons for each plugin requiring an update - however, after updating, the plugin manager still reports that there is an update available. I tried clearing out the dokuwiki data/cache/* directory, which did not help. I also verified that the plugins were successfully installed (they all were).
Sorry - completely wrong Category.... If someone could re-assign to an appropriate category, that'd be great - thanks. :-)
The plugin manager currently isn't that intelligent and simply doesn't know whether there's a new version of a plugin available or not. Thus, the update button always is displayed. However, it would be a cool addition if the manager could at least guess at such data (e.g. by retrieving the modification date of the plugin package from the server or something similar).
I'll change this to a Feature Request.
forthcoming extension manager will resolve. plugin manager won't be further developed.