Saving the environment is a task that's too difficult for one person to understand, so anything that seems slightly inconvenient is good enough to satisfy most people's "needs". Changing when you charge your cell phone is a lot easier than planning your life to be lower-impact.
(I love all the SUVs filled with one grocery bag at Whole Foods. Do you really think that buying some locally-grown organic carrots offsets the energy you spent driving your military vehicle 3 blocks?)
"Forget Shorter Showers" (http://www.orionmagazine.org/index.php/articles/article/4801...) points out that perhaps we've duped to think that energy shortages/waste are _our_ fault, when, in fact, it's industry, large-scale agriculture, and our military that are the biggest abusers. Of course, this isn't an excuse to not be lower impact ourselves (and perhaps not purchasing products that support certain industries), but we can't be fooled into thinking that changing our charging habit is going to save the environment.
For more from the wonderful Jensen on how abusive our culture is, read A Language Older Than Words (http://www.amazon.com/Language-Older-Than-Words/dp/189395603...). If you can look past the absurd stuff about talking to animals, there is a lot to be learnt from it.
if our changing habits were coupled with our buying, lobbying and voting behavior, then it would have a significant effect. furthermore, the culture and ethics we instill today will be in charge of corporations in a few decades.
Consider your energy sources too, here in ontario my power is coming from a mixture of Hydro and Nuclear. Plugging anything into my car to charge is most definitely worse, as you're then trading relatively clean energy sources for a car engine.
If your power comes from a coal/oil plant, it's a rather harder decision, which depends on your vehicles efficiency. However, it's most certainly going to use more fuel (and thus cost you more) than it would take to receive the equivalent electricity.
A commute long enough to charge your electronics is probably a bigger "sin" than a cell phone plugged in overnight.