I think this update will depend on the ongoing Apple litigation.
Apple's already trying to convince the world that HTC didn't meet the requirements in the ICS model we're enjoying now - and has since gone after Google directly claiming a new set of infringements in basic JB itself.
HTC's not going to push an update that takes them further into court because new models will come with the OTA baked in shortly after the update hits our shores.
The foodchain that was previously - Google creates -> later puts code in AOSP -> HTC retrieves code -> HTC updates -> Sprint approves and pushes OTA ... just got a number of places for Apple to target.
I don't want to start another round of Apple hate, but I see the above as directly on-point to the question in the OP - when are we going to get JB?
Because after the last few days, we have to decide - how many players this time around - before we can make those guesses.
Well, for me, anyways.