What type of file are you appending to? .TXT files? Cause it works fine for me. And are you using the Append To... or Append Text... action? Cause Append To.. is the only one that works for me, I can't get the other Append action to even come up.
Unless the TextWrangler files are a different format than .txt then I don't know why TextEdit files don't work. What Application is your default for opening text files?