The old behaviour (creating a file with mode 0666) may be a security risk, especially since other Unix flavours use 0600, and somebody might overlook this detail when porting programs.
More generally, the POSIX specification does not say anything about file modes, so the application should make sure its umask is set appropriately before calling mkstemp.
BSD 4.3, POSIX 1003.1-2001
The prototype is in <unistd.h> for libc4, libc5, glibc1; glibc2 follows the Single Unix Specification and has the prototype in <stdlib.h>.
mkdtemp(3), mktemp(3), tmpnam(3), tempnam(3), tmpfile(3)