The first lines of cdb.c and cdb.h read "/* Public domain */". I would expect DJB to spell this out more explicitly if he releases an updated cdb package. I say "if" because the last release was Feb 2000.
I hadn't noticed that. Wikipedia explains the licensing in more details (http://en.wikipedia.org/wiki/Constant_Data_Base#Library) - the library code is public domain (and included in the public domain djbdns), but the rest of the package is under djb's annoying "You may distribute unmodified copies" license.