Open Firmware Password complexity
Apple Support Document HT1352 states:


  Use a secure password that contains both numerals and letters in the
  first eight characters.


I assume from this statement that OF ignores the 9th and succeeding characters in the chosen password, but I haven't tested to see if this is the case. Further down in the document, an image of the Firmware Password Utility window states unambiguously that passwords composed of letters and numbers only should be entered: