Even though I am still having trouble getting bitcoin to work on NetBSD, I have been able to build it thanks to this additional Makefile (therefore called "makefile.netbsd").
I believe it has enough differences to "makefile.unix" to justify a duplicate (no libdl, packages in /usr/pkg by default, libdb_cxx is called libdb4_cxx...) but let me know if you'd prefer to adapt "makefile.unix" with conditional building instead.
Cheers, -- khorben