Maclife suggests ferrite beads as a fix for the GSM buzz emanating from speakers when placed near a busy cellphone. I found some of the snap variety you can order from Radio Shack, but am unsure of this trick working when it comes to situations where the entire length of cable is receiving interference, or on an input cable like that on a tape adapter where the signal eventually gets amplified. [MacLife, via smashgods, Radioshack]
