Contrary to what I had thought the process was to be, the JP cap is now in effect at 4,000 points. If you had more than that, then you will have got an in-game mail with some gold by way of compensation.
I had thought it was to kick in on MoP launch, so it's just as well I spent it all on buying one or two of every heirloom going in the past few days!
ROFL, just logged my shaman and got over 5k gold from the conversion!