Making sure that DNS is resilient is also important so it does not become your single point of failure (SPoF), if you host you own DNS ensure it is hosted on 2 geographically dispersed locations or use a resilient DNS provider (e.g. Dyn). If your DNS is down you may not even be able to send an email notification to you customers!

As the post infers, make sure your status site e.g. "status.domainname.com" is hosted on different infrastructure (even using an alternative ISP), this will ensure it is always available.

While working at my previous employer I hosted the companies status site on their corporate infrastructure (as it was a low traffic site), keeping the clients sites hosted in a Tier1 data centre, DNS was with Dyn so we had eliminated all the important SPoFs. This meant that communicating with clients during an outage was not (generally) an issue.