The 6.x-1.1-dev version of the TinyMCE module doesn't work when used in a CCK text field set to "unlimited" for two reasons. One is that the "Add another item" button uses AHAH and the tinymce module doesn't handle that well. This module enables the tinymce module to play well with AHAH. While the most common AHAH use-case for it is within a CCK field set to "unlimited", this module also enables tinymce usage in other AHAH scenarios.
The other reason tinymce doesn't work well in a multiple CCK text field is due to integration problems with drag and drop. This problem is addressed in a separate module: tinymce_dragdrop.
This module depends on both the tinymce module and the jquery_form_update module.