dmor, really? It looks like the blog points to AWS, as does your API?
jdyer@aleph:~ [git:master] <ruby-1.9.2>
» host api.twilio.com
api.twilio.com is an alias for public-vip374d1ca4e.prod.twilio.com.
public-vip374d1ca4e.prod.twilio.com is an alias for ec2-174-129-254-101.compute-1.amazonaws.com.
ec2-174-129-254-101.compute-1.amazonaws.com has address 174.129.254.101
----
jdyer@aleph:~ [git:master] <ruby-1.9.2>
» host www.twilio.com
www.twilio.com is an alias for public-vip29c4ab3d.prod.twilio.com.
public-vip29c4ab3d.prod.twilio.com is an alias for ec2-174-129-253-75.compute-1.amazonaws.com.
ec2-174-129-253-75.compute-1.amazonaws.com has address 174.129.253.75
DNS lookups don't tell you anything here. The way a reverse proxy works is that HTTP requests to certain URLs get turned into an HTTP client request by the web server to the 3rd party provider (for caching, URL changing, compressing, terminating SSL, and get around firewalls). You can learn about them here: http://en.wikipedia.org/wiki/Reverse_proxy
True, and I had actually misread dmors' post entirely here; I read the post as stating Twilio's blog was not reliant on AWS in anyway, which would have been a misrepresentation in my mind. However in hindsight this was not the case, and I will certainly admit when I am wrong.
