DETROIT, Jan. 7 — The virtual ballot box has arrived in Michigan. Democrats in this state are the only voters in the country who have the option of voting online in the presidential primaries this year. Since New Year's Day, voters have been allowed to apply for ballots and vote by mail or Internet in advance of the Feb. 7 caucuses. Or, on Feb. 7, they can go to one of 576 caucus sites and vote the old-fashioned way. By Thursday night, 11,000 people had applied for ballots, three-fourths of them over the Internet, according to the Michigan Democratic Party....