The frequency of the signal has something to do with how noticeable it is though, at least from my experiences. I had a Sony Ericsson T610 on TMobile that ran on the 1900mhz band. A friend of mine had the T616 on Cingular running at 850mhz. Pretty much identical phone internally except for the radio. The T610 could use 900, 1800 and 1900mhz. The T616 was 850, 1800 and 1900. The notable difference? His would cause the noise on a ton of speakers while sitting in his pocket, including overhead PA speakers at his work if you listened closely. I'd have to put my phone practically ontop of unshielded wire to get any noise out of the speakers.

It's not anything I was ever concerned about until I switched to Cingular/AT&T a few years ago. Now I have to make sure my phone is across the room when using voice chat on my computer, because the mic input on my headset or USB sound card isn't isolated well enough against it, and the people I am talking to will hear it quite loudly. My speaker lines seem to be shielded well enough on the headset that I won't notice though.