I assist other departments on campus with Mac related issues fairly regularly, since I’m one of the few Sysadmin’s on campus that really know Mac OS X Server. The issue they were seeing (and have been seeing since they upgraded to 10.6 about 4 months ago) was any time someone tried to login to a client, or really anything as a user that was part of the OD, it would take about 60 seconds to authenticate. If they used their server’s local admin account, however, it worked instantly.
Everything seemed to be running, but it just took a long time. Investigating further, everything seemed to point to Kerberos just not functioning. It was running, but kinit would take about 60 seconds to come back asking for a password. And for some reason, the REALM for the Kerberos server had been set as SERVERNAME.LOCAL. Which, shouldn’t be an issue in of itself, but it was certainly not “proper”.