1] The top screenshot is pretty "jarring", which I'm sure was you intent and works well.. However, it doesn't indicate what happens when the ssl cert is invalid & users would see this screen. I think it would be much more informative to add an arrow pointing to an email inbox or something indicting that you get alerted when this situation happens. Additionally you may want to change the site url to "www.mysite.com" or something - If I was Amazon and saw that screen, I wouldn't be very happy.
2] Email is great, but you gotta add cell phone (sms) support in some capacity. You can do this on the cheap by just offering support to the major cell providers via their email-to-sms gateway instead of using a paid service like Twilio or Tropo. It's not as clean, but it's cheap and easy. You just have to have the user put in their cell phone & provider and just send a much shorter alert notice. Here's a good list of email-to-sms gateways: http://en.wikipedia.org/wiki/List_of_SMS_gateways
3] If I were going to use this service, it would be important for me to know how often your service is pinging my sites to verify SSL cert integrity - once ever minute, once every five mins? Couldn't find this on the page or FAQ
Otherwise this is a great idea! I can definitely see the usefulness of a service like this, and I wish you great success!