In the 10.6.3 version of ServerAdmin, the checkbox to REQUIRE authenticated binding to your OpenDirectory master is gone. You can USE authenticated binding, but not REQUIRE.
Here is the command to require auth. binding via the command line:
sudo slapconfig -setmacosxodpolicy -binding required
See the knowledgebase article http://support.apple.com/kb/HT4068 for more info.