It seems as though if the droid is too hot it simply won't charge. I noticed this happening today when I was using navigation while plugged in to my car charger, mounted in the car dock. Today was HOT, 90+ degrees F, though I had the AC blaring. Basically the droid began charging when first mounted, then I noticed it stopped for no reason. The temp of the phone was around 55-59 degrees C since it was sitting in the sun on my dashboard. Once I took it out of the car dock, turned off navigation, and held it in my lap for awhile to let it cool down, it finally started charging again.
Perhaps this is a safety precaution to not heat the phone up anymore, but it really sucks when you need navigation on a long trip during a hot day.
Anyone else experience this? I'm on Cyanogen 5.0.7 overclocked to 1ghz for what it's worth.