The iPhone's GSM radio can cause significant audible RFI near speakers that aren't sufficiently shielded. Example: Apple's original Design Speakers from the early 90's were designed back before such radio interference with cell phones was an annoying issue. Tried adding various ferrite beads to every end of every cable connected to the speakers, but never found an effective attenuator against 1,900 MHz.
Then came across the poorly named QB-EM Shield by Quick Bridge Solutions. It's a business-card sized piece of laminated material that appears to resist GSM 850/1900 well enough to reduce the iPhone speaker buzz to near inaudible levels without totally blocking the signal for incoming calls. Simply place the iPhone on top of the QB-EM Shield, and all is well. The QB-EM Shield looks homemade, but it's hard to complain at only $4 a pop plus $1.35 for shipping. Bought one for the office desk with the Apple Design Speakers, and one for the "ashtray" where the iPhone sits in the car.
