Shoutbox

Also, in order to be able to do your idea (which has been suggested in a slightly different way, like changing the green buddy icon to something with a Plus! logo), you need to know who has Plus! installed.

In your way, Plus! should send data to get this information in a hidden way - people will not know they have been checked to see if they have Plus!. This can be considered a breach of privacy and will therefore not be added.