I found the answer to my question. SYNC 3.5.1 is apparently ready and available for FFE and PHEVs running 3.1.3. A user (cmax driver) on a Sync support community board posted a successful update yesterday. I confirmed that an update is available for my VIN, and then my dealer's service dept confirmed my confirmation. They haven't done any of these yet so the tech is calling to get the process instructions before scheduling my update (it has to be done by the dealer.)
Anyway, you can check your vehicle's VIN on etis.ford.com, clicking the "vehicle" tab on the top of the page. If the update is ready as I expect it should be for everyone, it will be listed as an FSA or Field Service Action, "13A01 Sync with my ford touch- application performance upgrade".
Anyway, you can check your vehicle's VIN on etis.ford.com, clicking the "vehicle" tab on the top of the page. If the update is ready as I expect it should be for everyone, it will be listed as an FSA or Field Service Action, "13A01 Sync with my ford touch- application performance upgrade".