Greetings, all.
Another 2 cents, maybe 4..... I am the Freq. coordinator and membership sec. for one of the clubs I belong to.
Due to unverified rumors, "No Cell" signs were posted at our field as well..
I and some other members questioned the rule, and My investigation has thus far come up with NO hard evidence that the issues exist.
I have a quote from Steve Kaluf at AMA, as mentioned above, they were never able to duplicate the Programmable Tx Issue.

Now, as for interference, being an RF technician, I want everyone to be aware that it's more than the specific frequencies that can cause issues.
Yes, cell phones, CB's ham radios etc and etc are on totally different bands. However it is possible, even commonplace, for multiple RF signals to mix. Your receivers also use an Intermediate frequency, usually 10.7 MHz, in the process of extracting the servo commands from the RF signal.

I don't want this to be too long and drawn out, but if there is a pager tx nearby, that happens to transmit on 167.38 MHz, take the first sub harmonic, 83.69MHz, subtract the 10.7 MHz IF, and voila, I have interferance on 72.99 MHz which is channel 60.
Now, for every RC freq. there is a 1st harmonic (twice the freq) a 1st plus IF, and a 1st minus IF where it is EASY to create interferance!!!! If there happen to be a few transmitters in the area, they can mix signals that would otherwise not be an issue and cause interferance.

For the record, I am building a directional antenna for the 72 MHz band, because a fellow member and I were both shot down on 60 the same day. On BOTH occasions, we turned off our Tx's and turned on a scanner and found a STRONG signal, definately RC, on channel 60!! This is Seconds after the crash!!!!!!
This is an AMA club site, with a few farm houses in the area. My suspicion is that someone was testing a radio system in the area without either knowledge or regard that there is a field nearby.

Lastly, another club I belong to has a cell tower ON THE FIELD!!!!!!!!!!
Thus far, (Several years) there have been no apparent issues with the tower or cell phones on the field.
Again, to err on the side of safety is my preference, but I think INFORMED fliers can have MORE fun!!!!!!