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Using Drupal 7.20, The latest WYSIWYG release has no more teaser break icon in the toolbar. I upgraded to the latest TinyMCE, which did not help.
The last version of WYSIWYG I had installed, which works, is here:
; Information added by drupal.org packaging script on 2011-06-19
version = "7.x-2.1"
core = "7.x"
project = "wysiwyg"
datestamp = "1308450722"
I reverted to this last WYSIWYG release and the icon has returned. I know that there is a summary option in D7, but it is difficult to get people accustomed to using it. The teaser break is the best option for end-users, IMHO.