Measuring Input Impedance of Transistor with LTSpice
So from this plot it looks like it's about 2.9K. Is this correct?


Yes


  Obviously with the bias high enough the impedance of the transistor is 0 - it's just a diode. Right?


Probably not. The diode model probably includes parasitic resistance in the base and emitter contacts, so the input resistance will never go all the way to zero. Also, your R3 (multiplied by the transistor beta) will set a lower limit on how low a resistance you measure looking in to the base of the transistor in your circuit.