Depends on what the cost of that power would be. A large overnight demand would drastically change the economics of power generation.
Furthermore, millions of plug-in vehicles is in essence a huge distributed storage system: one big national battery. In this scenario wind power would be, by a significant margin, cheaper than coal and nat-gas electricity.
Bingo. Wind is already #2, behind gas fired turbines, in new electrical generation. In high resource areas, it is now the cheapest marginal source. (Yes, kiddies, 30 years of government subsidized research has matured the technology.) The issue with wind for baseline power generation is reliabilty and storage. Plug-ins are a good fit.