I had this conversation with Mitel and was told that the integration with S4B is a direct SIP integration(SIP Trunk between S4B and Mitel). You are required to have mediation server deployed in your S4B architecture.
We're talking about two different scenarios here:
1/ Build a Skype call server, configure a SIP trunk between your Skype call server and your Mitel controller.
--> this allows Skype callers to call numbers that can be routed to the Mitel controller so that Mitel callees can receive the call.
It's basically like having two in-house PABXs trunked to each other with one group of users on each of them.
I've configured a SIP trunk between Mitel and a Skype call server (forgot what it was) and it works fine for routing calls between the two different systems, although the Skype system is fairly functionality-poor.
2/ Using a single voice system - a Mitel controller - allow some/all of your users to use Skype to place calls on the Mitel system by installing the Mitel plugin which adds an additional "dial" option to the Skype drop-down menu action list. This will suit users who don't want access to MiCollab profiles, statuses, calendar integration, replay voice messages, integrated access to AWV, and all the other functionality of the MiCollab Client.