[robotrout]

Expanding on that idea a little. Store the proof of operation on the clients own server.

Let them enter a url to a script on their own server, that you will POST to, whenever you check their server. The user can log POST's to a text file, DB, etc., so they know that you are actually monitoring them.

When they see that you haven't POSTed to that file for a while, they know you're out of business, and they are unprotected.

On second thought, make it easy on yourself, and use GET. It's not like there's anything confidential about it.

[uggedal]

I've been thinking of implementing webhooks as an additional form of notification, but not as a a presence mechanism. I like the idea, but given the simple nature of the service I'd have to give it some thought before implementing something like this.