I've been using a Belkin TuneCast (FM transmitter) hooked up to my Palm Treo for about a year now (off and on).  It's a great little add-on for converting my PDA/phone into an mp3 player that can play music through my car's sound system.  Initially, the obvious problem was that it's only capable of transmitting on 4 different radio frequencies, which means there's a chance it won't work in heavily radio-frequency-populated areas (i.e. cities, Best Buy [kidding]).  But recently I've discovered a new problem:  Interference with FM transmitters in cars driving past me.  At first, I was wondering why I kept getting a second of seemingly random crystal clear radio stations while my songs were playing.  I figured I was driving through tiny little areas of radio station coverage.  But then I realized that it only happened as I drove past other cars.  That's when I figured it out:  Other people's mp3 players hooked up to radio transmitters were interfering with my transmitter.  That made me a little mad, because I'm not quite sure how their transmitters are more powerful than mine.  Maybe theirs are cigarette-lighter-powered?  Mine is battery powered.  Maybe mine isn't close enough to my antenna?  Does the passing car hear my music as I'm hearing theirs?  I doubt it.  But the whole thing gets me a little angry and makes me go back to CDs, my over-sized, under-packed wastes of money. #entertainment