For the record, I don't believe this impacts custom domains. I did a few searches on some of my custom domains and they return things like the following response message:<p><pre><code> We haven't information about @yourdomain.com</code></pre>
Quite useful, thanks! It looks like custom domains are in fact not affected. That's a relief. However, with the vast amount of accounts GMail must have, 5 million accounts seems like a small number. Lucky I wasn't in there.