You can freely incorporate public domain code into a code base licensed under any license whatsoever. For example, as the FSF states in http://www.gnu.org/licenses/license-list.html, "Public domain material is compatible with the GNU GPL."

For the other direction, that is, incorporating some copyrighted code into an otherwise copyright-free code base, just be careful to keep the copyrighted files isolated and clearly marked as encumbered, so that nobody accidentally copies anything from them into the unencumbered portions of the code base. See http://groups.google.com/group/unlicense/browse_thread/threa... for more information.