When the US car market includes 50M plug-in hybrids, I’d guess at any one time 5-10M of them will be plugged in somewhere. A tiny fraction of that total capacity would provide all the grid power storage that is needed. Utilities might even pay the car owner a few cents for every KW it “borrows”, encouraging owners to keep their cars plugged in when not in use. The metering and micropayment system would be easy to set up.