Sea Six asked me earlier if this was possible, so I looked around and discovered it was, and added the feature here.
You can now customize which smilies appear in the reply box in posts and PMs..
-Go to
User CP
-On the left menu, under "Settings & Options", select "Edit Favorite Smilies".
-Select up to 24 smilies, then save.
(for obvious reasons, I'd suggest not choosing any of the large ones).
Enjoy!