It's not relly the speaker itself. Here's the mechanism:

The speaker coil and the cable from the amplifier act as an antenna. The radio frequency signal (RF) is led thereby to the last stages of the amplifier, and often from there via the gound of the system to eariler stages.

The semiconductors in the amp convert some of that radio wave to audio noise, and that noise gets amplified back and presented to the speaker.

I am clarifying this to calm some worries. If it was a matter of the radio wave exciting the speaker directly, there would simply not be enough energy to be audible. Without knowing this fact, people might assume that there is a whole lot of dangerous RF energy from the wireless device, when in fact it is a very tiny amount.

A technician cam remove this interference completely, not best by huge amounts of shielding, but by adding a few bypassing componemts at the output of the amp, TV, or other audio device.

OH, BTW, in the case of the auditorium sound system or a component audio system, it is just as likely to happen due to that energy coming into the inputs, like via the microhone cable or the interconnecting cables. Same bypassing rules apply.

For technicians wishing to solve this problem for their equipment, I recommend the ARRL publication about Radio Frequency Interference. (title?)

As far as the seriousness of the problem, it is low to medium importance at this time, but will get rapidly worse as wireless technology prolifertes.

I lay this directly in the lap of the FCC (here in America, anuway.) Equipment can be designed to minimize susceptibility to interference, but since it is not something that can ber determined to be a problem in the electronics showroom, some sort of susceptibility rating should be mandated to be reported "on the box", if not a mandate requiring better resistance to interference.

The added component costs are trivial. Remember, it's not the fault of the transmitters, but the existing receivers.

Out of curiosity, I did a few searches for aftermarket interference-limiting devices and surprisingly I found very little. There should be RF-filtering audio adaptors, speaker filters with quick connectors, and the like. There are a few, but not in the mainstrean electonic supply, other than the snap-on ferrite chokes at Radio Shack, which are not terribly effective for this type on interference.

I really expect this to change, though, as the problem gets worse. Also, fiber optic interconnects for audio equipment are becoming more common, which are completely immune to RFI.

Footnote: Corporations play games with regulation, too. A few decades back. a major computer manufacturer in the UK introduced their education-oriented machine into the US. Their US competitors managed to get the govenment to specify strict noise output regulations on the imported product, but not the domestic competitor. The shielding required easily added $50 to the foreign product, which amounted to a hidden import duty. (This was before PCs were as universal)

Mostly, it seems to me, the FCC responds to corporation's needs rather than those of the users.

Vince-
I recently read that the cylindrical magnetic things that come on many USB cables can stop this buzzing if attached to the car speaker cable near the speaker. I've been collecting them to try on my wife's Acura, which has a buzzing problem with her iPhone 3G. Her Motorola Razr was much worse. Are these the Ferrite Chokes that you speak of? If so, I won't bother trying them.
On the other hand, my Mercedes, which uses a fiber optic loop between all the electronic components has no buzzing problem with my iPhone 3G, which confirms your point.

Thanks,

Kirk