All Spektrum Transmitters and Receivers are backwards compatible with DSM2 with the exception of the first DX6 (without an i).
A warning is that not all "pirates" called DSM2 are 100% DSM2.
If i am getting a DSM2 BNF plane from HobbyKing today i would not expect any problems at all binding and flying it with any old or new DX6i transmitters.
I use DSM2 and DSMX radios with my orange Rx receivers for most of my models and have had almost no trouble. I say almost because the 9 channel i'm using in my Hobbyzone Supercub has locked up on me twice, but I always got it back before it crashed. I believe that this is caused by the old servos I'm using. I suspect that the numerous crashes in their lifetime have caused them to overdraw the ESC on occasion. I use the 6 channel Rx's in my nitro heli even and have had no problems at all.
The DSM2 and DSMX radios are compatible with the DSM2 and DSMX receivers, because they will revert to DSM2 whenever DSMX isn't present in both. So, a DSMX radio will work with a DSM2 RX and a DSM2 radio will work with a DSMX RX.