At the risk of starting a flame war, I'd say don't listen to the fearmongers. Headphones while riding is fine. You can hear traffic even with both in -- especially if you're listening to spoken word, instead of constantly-pumping music turned up to 11.
I listen to spoken word/podcasts/radio shows etc exclusively, and I do find it works well to listen right-ear-only (if you have a half-broken pair of buds around!)
Also I recommend silicone-tipped earbuds, rather than solid plastic jobbies. At least for me, the solid plastic ones don't like to stay in -- or maybe I just have freakishly-small earholes.