in my app it works...
i've got TinyMCE and rich:calendar on the same page and all works...
my work on tinyMCE is described in this post :
it use it with ajax and jsp_compressor...
I have found workaround this problem.
If you move all javasript initializing tinymce from head after the element for rich:calender or just before the text area it seems to work correctly.