Apple's 10.2.3 update correctly set the permissions for the root directory to read+write for the owner (root), read only for the group (admin) and others. But if you downloaded, for example, the Quicktime 6.1 update, it set the permissions back to read+write for group, which causes sendmail to throw a wobbly.
Probably other Apple updates would do the same.
It looks like repair permissions also does this, but I don't know what would happen if you just loaded the 10.2.3 update and then ran repair permissions (ie before allowing the Quicktime update to change them yet again).