To be clear, the source code for the website http://raspberry.io was open sourced. (Not any more source code to the embedded device itself)

Github link: https://github.com/python/raspberryio

This is pretty awesome. I don't know that Pi was "meant to be paired" with Python but it certainly seems to be a nice match. I have a feeling most of the development on this device will end up being Python stuff.

Cool site, I just signed up!

list of projects http://raspberry.io/projects/

Does this mean we have the source code to the TrustZone SecureOS?

No -- they just open sourced the website code.

Does the rPi even do anything with TrustZone?

The question is, "does TrustZone do anything with rPi?"