Finally got Bluetooth syncing from my One X to my Fitbit Flex, here’s how –

First ensure that your phone has at least Android 4.3 or higher, in my case I’m running Cyanogenmod 10.2, then head to this link to sign up for the Fitbit Beta – https://groups.google.com/forum/#%21forum/fitbit-android-sync-beta. Once you sign up to for beta access it may take around ten minutes for this to register with Google, then an update to the Fitbit application will be available on your phone.

After the update ensure that Bluetooth is active and enjoy your wireless syncing!