The applet does not send any information to the server until you "Save
to public gallery." Therefore, you may play around with your private
texts, print out the resulting Wordles, or take screen shots, without
compromising your private information.
When you do save to the Wordle public gallery, only the information
required to reconstruct the Wordle is saved. In particular, I'm saving
a list of words and their relative frequencies. None of the source
text is saved as such.