Pocket Wizard today announced firmware update version 3.707 for their MiniTT1 and FlexTT5 transmitter and receivers. The update now fully supports the D750 as well as the SB-500 Speedlight. Note that the SB-500 cannot be used as a master flash, only as a remote flash on a FlexTT5. In addition, SpeedCycler has been improved.
Note that when you update your Pocket Wizards, you need to make sure everything is on the same firmware release, and you should perform a factory reset of the radio after the update to insure that it comes up correctly.