What is the nominal frequency of the satellite communications terminal on MH370?
I looked through the report about the flight path analysis that is available from this page.

In Appendix G they explain in more detail how the analysis was done. On page 56, they provide the details about the frequencies (emphasis added):


  The uplink and downlink Doppler may be calculated from the relative movement of the aircraft, satellite and GES using the signal frequencies of 1646.6525 MHz (uplink) and 3615.1525 MHz (downlink).


This means the aircraft terminal transmits (uplink) on 1646.6525 MHz and receives (downlink) on 3615.1525 MHz. However, it is not that simple.


  The aircraft terminal adjusts its’ transmit frequency to compensate for the Doppler induced on the uplink signals by the aircraft velocity. Aircraft heading and ground speed are used to calculate the Doppler shift the signal would experience if the satellite was at its nominal location over the equator.


This is called Automatic Frequency Control (AFC) and Appendix G goes into more detail about all the factors involved, and provides tables with example calculations.