I thought it doesn't charge when it's powered off? Or at least mine doesn't I don't think.
Question is: Why the heck are you powering off your iPhone? Mine is on 24/7. I only reboot it every once in a while if it starts to slow down.
I charge mine when I don't want it on. (ie. Late Saturday nights) And my iPhone 3G as well as my iPhone first gen charge while powered off.