I believe that the Model X reveal is being held up because of new battery technology. If outed too early the current backlog of 10,000 Model S orders would be diminished and many orders would wait for the new battery to be added to the Model S in the fall of 2015 (remember, no changes until then). Imagine if TODAY Tesla showed us the new Model X with the 100 kWh battery (or 20% lighter battery). The Model S85D would have a 350 mile range (yes, I did the math). The P85D could have a 0-60 in 2.8 seconds (a pure guess) and beat the Corvette Z06.
Side view cameras, improved processor, LTE, etc would also trickle down to the Model S. Everything but the Falcon Wing Doors.