You should at least know there is an outage to have something to tell your downstream customers. It is really embarrassing to have a customer (or your boss) call to report an outage you don't yet know about, even if there is fuck all you can do to resolve it. Basic principle of ops.

That gets done for network/application performance monitoring (alternatives to keynote, gomez, etc., and is how some of their own products work). It's kind of overkill for basic application level monitoring -- there's a tradeoff between number of endpoints checking and frequency of checks. I guess you could round-robin checks across a larger number of end nodes, too, to get both.

Do you work for a DNS provider or CDN or something (so as to see this in near realtime)? Envy.

I haven't seen a lot of people using both EC2 and Terremark for the same app -- kind of different markets. Not technically unreasonable, but Terremark seems to be more enterprise IT outsourcing, and EC2 (followed at very far remove by the other clouds, including Rackspace) being Internet-delivered consumer, etc. apps, or at least larger scale public services.

Me, i use specific load balancing for my trafic when Cloud outage is detected. And i sleep perfectly ;-)