The newer versions of Preview (at least OS X 10.5, and maybe earlier) respect the “do not print without a password” setting in PDF files. However, it is possible to create a completely unprotected version of any PDF file using ghostscript. A command like this should work:
gs -sOutputFile=MyNewFile.pdf -sDEVICE=pdfwrite -dNOPAUSE -dbatch -q MyProtectedFile.pdf
You may need to type “quit” at the command prompt after gs finishes processing the file. Then open the newly created PDF file and voilà, printable!