This happened with my B+W 20GB 4G. The right earphone on my Sony EX-71's just stopped working one day. I used some stock headphones for a few weeks while I sent the EX-71's back to Sony (they broke during the last week of the 90 day warranty), and they replaced them.
Got a black 5g 30gb shortly after and haven't had the problem since.
Got a black 5g 30gb shortly after and haven't had the problem since.