Accessory not supported by iphone message keeps appearing . . . .

This morning, my iPhone began to randomly display a "accessory not supported by this iPhone" message and keeps asking if I want to go into Airplane mode. On more than one occasion, the phone was not plugged into ANY accessory and was simply sitting on the table.

On another, it was playing music via the headphone jack into my car stereo.

I'm running V. 3.1.3, upgraded when it came out and have had no incident until today.

Any ideas? Not a big problem, just more of an annoyance.