The 16gb J3 is on newegg.com right now (not sure about amazon) but a 32gb version has just come out in Korea, so maybe you want to keep waiting. Me, I have a D2 and prefer it to the J3 because of its full sized SDHC slot and its standard mini-USB connector instead of a proprietary cable. I think its battery life is longer too, and its SQ should be pretty close. The J3 has a nicer screen but I don't much care about that.
The USB port wants 5 volts and a cell phone battery is 3.7 volts (like the internal battery). Also you'd be messing with an external battery and cable if you did that. My idea is to just mount the phone battery on the Clip with double stick tape, running a wire into the Clip through a small hole drilled in the back, so there would be no extra plugs or cables sticking out.