MBPro is unable to send to WeatherLink.com with or without a subscription. I would assume MB PI to be the same? A Nano/SD can upload to WeatherLink.com without any subscription, requires a simple account.
@Mattk: Are you sure about this - it's not obvious from the MB website? (Not doubting the comment at all, but it's a detail I wasn't aware of and would be useful to clarify 100%.)
If accurate then it presumably implies that MB doesn't itself currently synthesise the current conditions packet for upload to weatherlink.com. I can't imagine that this would be a difficult feature to add but that's obviously for Boris to decide. Following this train of thought, it sounds to me like there are two scenarios:
1. If MB sources data from a Vue/VP2 console (Nano; MBfPi with a USB logger or WLIP-type feed; and ditto for MB Pro Red or Black) then it can upload to weatherlink.com
2. If MBfPi sources data from WLL or directly from the sensor transmitters (Red) then it can't upload to wl.com
But this is just me speculating and official confirmation would be good.