I figured out my issue...remember - it always helps to read the documentation.
I'll outline my mistake here so that others won't fall into my trap.
Here is how I was configured:
PSTN -> Media Gateway -> Mediation Server -> OCS
I was expecting inbound calls from the PSTN to hit the OCS user, but then spin the call back out to the Mediation Server since the user was PBX-enabled. The next hop on the Mediation server was to my IP/PBX.
That is not supported nor expected behavior. Dual forking for OCS will only occur if the a call is made to an internal user from inside OCS. When OCS pops the internal user's client, a new call will be spun out Mediation Server to wherever your next hop is (typically a PBX, Proxy, etc.)
For dual forking to work the way I expected, your architecture should look like this:
PSTN -> Media Gateway -> PBX -> Mediation Server -> OCS
and the PBX is responsible for forking inbound calls to OCS - whoops.
Luckily when your company is the creator of the IP/PBX you're using it's easy to adapt/test/get it working; which I did today.
Sorry for the noise - I just figured someone else would do this.
--Matt