Actually, coming from an ex-Radio Shack employee of many years (I even managed four different stores), what Michigan recommends, in combination with a mono earplug like 55/Rad recommends, is exactly what you want. The adapter Michigan points out is a stereo to mono adapter -- plug that into your iPod, and plug the mono earplug into it. Viola! Both left and right channels will mixed into the mono earplug.

Granted, you won't get stereo separation, but you're only using one ear anyway. You may also want to hunt down a little bit better mono earplug as the one Radio Shack sells is kinda cheap and tinny. But for a quick fix, that's yer ticket.