From the FAI Rules :
...Radio equipment shall be of the open loop type (ie no electronic feedback from the model aircraft to the
ground except for the stipulations in Volume ABR B.11.2).

ABR B.11.2 states:
A Spread Spectrum technology receiver that transmits information back to the pilot-operated transmitter,
is not considered to be a “device for the transmission of information from the model aircraft to the
competitor”, provided that the only information that is transmitted is for the safe operation of the model
aircraft.


So the bottom line is that according to the FAI, it's illegal (while people argue with the CD over what's just for safety and what's helping the flight). However as has been stated earlier, rules in your country and what is actually enforced at local level probably will be different and no CD will turn you away.

Two sides to this funny coin though, back when the 10cc (two stroke) limit was in there was no point OS bringing out a .65 sized pattern motor, so why should the rules be changed (or a blind eye turned) to allow Futaba/JR/Spektrum Radio systems if they don't comply with the current rules?? Should Futaba/JR/Spektrum be made to provide an "F3A" model option in the menu which disables the illegal funtions for that particular plane. Electronically "sleeve" the radio down, very easy for them to do.....