Shoutbox

Manually checking each and every user, querying if they have plus installed, is really very cumbersome.  Realistically, taking into consideration the way the system works now, if you want to know if someone is using Plus!, you may as well just ask them!  So yes, a better solution is needed.  However there are difficulties to be overcome; technical and privacy.

The technical aren't the main concern, the privacy is.  Personally I sit amongst those who think by installing Plus! you should really be allowing people who satisfy the following:
a) Have Messenger Installed
b) Have plus installed
c) Are accepted on your contact list
d) Are not blocked
...to see you have Plus installed (here comes the critical bit) via a SILENT HANDSHAKE.  It should be presumed that having Plus! installed (amongst all the other conditions listed above) would allow this.  The same underlying core principle is applied to Messenger itself.

This would be beneficial to seeing what features could be activated successfully (I.E. What's the point of sending people formatted text if all they can see is a heap of junk characters?).

However this is only one side of the story, and those who think that a silent handshake model shouldn't be allowed bring up their own set of valid points, much of which I also agree with.

This is a hard issue to place.  A better solution is needed, but (ultimately) compromises will need to be made.