That doesn't do anything for me (btw it should be option-command-t). That is even in iMessage.To access Native Emoji in Lion or later, go to Edit > Special Characters or Ctrl-Alt-T. The shortcut doesn't work in all applications (it hides the address bar in safari, for example), but it does in iMessage and probably many others.
I use Character Viewer for emoji on OS X but it is kind of pain.
Michael