Kerberos creates shared libraries, and installs them as expected in /usr/lib. However Cyrus-IMAPD is expecting des support to be found in 'libdes.so'. Kerberos [now?] writes 'libdes425.so'. I made a link w/ the old name, and Cyrus seems to compile just fine now.
cyrus-imapd-2.1.9-r2 has been added to portage. Please try it out and let me know if kerberos detection now works for you.
cyrus-imap*-2.1.10 has been released into portage as well with the prior fixes.