I'm looking for an overview and requirements on how this will work... including
Root CA selection (across the platforms)
What does the request look like?
How does the payment "flow" look like?
The main benefit of the Bitcoin Payment Protocol (seen in BIP 070) is to provide for a better customer experience and better security against man-in-the-middle attacks on the payment process.
See this BitcoinTalk announcment and information from the Bitcoin Foundation. The code for this is located in the PaymentServer.cpp file
How are Root CAs selected?
Root CAs are selected from this Mozilla root CA list. Although I haven't seen anything to verify this, but I hope there is a way to account for Certificate "extended key usage" in the PKI hierarchy.
What does the request process look like?
No GUI screenshot of this process is available (yet), but a workflow of the process looks like this:
