There appears to be a benign segfault from opendkim when Zimbra is shut down:

opendkim[4057]: segfault at 1f0 ip 0000003044209220 sp 00007fffa235bd48 error 4 in libpthread-2.12.so[3044200000+17000]
opendkim[10772]: segfault at 1f0 ip 0000003044209220 sp 00007fffbd4e47a8 error 4 in libpthread-2.12.so[3044200000+17000]

The only solid reference to this I could find was at http://www.zimbra.com/forums/administrators/58839-mta-logger-logswatch-wont-start-also-opendkim-issue.html. These users appear to be using CentOS 6.3 as well which may indicate the problem is with libpthread and not in fact opendkim.