I work as a sys admin at a medium sized medical research institute, one of the things I made sure to do was to add nagios scrips to throw alerts for important licenses, certs, and domain names. I'm not sure why an organization as huge as Sony Online would not have added these kinds of checks to whatever monitoring system they are using. Having had this happen to me once nearly a decade ago with a SSL cert I can promise that the 10min of coding to add in a check is much more pleasant than a day of meetings to describe to everyone what went wrong.
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I don't often recommend SUN products with the exception of Solaris but Sun Java System Directory Server Enterprise Edition has actually proven to be a very stable solution. I don't believe its open source but I believe it is free. There is also an identity synchronization tool that allows you to sync your LDAP to AD servers if needed. Handles multimaster replication between however many nodes flawlessly with very good performance in my experience. It'll run on Windows,Linux, or of course Solaris.
Good luck, LDAP is a pain in the ass