Some friends of mine had a similar problem. They wanted to use an ipod as a live back-up band (Lame, I know) and the click and music were hard-panned, like you're trying to do, but there was a problem. The electronics on the new Ipods bleed certain frequencies to both sides of the phones to give an "enhanced" experience. So in their situation, you could hear the click within their backing tracks when they played live.
You're using an imac, right? Maybe there's the same issue with it's sound driver. Maybe a similar issue with your phones. And if you're using itunes to listen with the sound enhancement feature on, then you are definitely going to have issues.
I meant to respond to the O.P. by the way