I created a clone of my main disk (centOS 6.5, with a fully running stack) into a new linode, added a swap disk on the new linode, created a profile for the disk+swap, and booted it up. The new linode is not accessible in any form or manner (http, ssh, etc). It doesn't even respond to ping.

Not sure even where to start how to diagnose this? I've created several clones before without any problem.

Enable the Network Helper on the new Linode.

https://www.linode.com/docs/platform/network-helper

You're a lifesaver, enabled it and it now works.

If you ever have trouble getting into a Linode in a situation like this, you can also use Lish (Linode SHell).