The Politico's Ben Smith has details of an effort started by some Jewish supporters of Barack Obama's; they're now raising money to buy a full-page ad in the New York Times that will show "the U.S. Jewish community's widespread support for Barack."
Thus far, Obama's had some trouble getting support from the Jewish community. Obviously, the Jewish community doesn't represent a major portion of the country's population, but Jews are a solid and fairly important part of the Democratic base.
From personal experience (and obviously, this is anecdotal and in no way scientific) I know that I've seen some evidence that whisper campaigns against Obama being conducted mainly through viral e-mail are having an effect. At various points, I've heard Jews who are otherwise solid Democrats expressing concerns about Obama, in part because of the blatantly false information -- usually about his religion, his purported connections with the Nation of Islam and his stance toward Israel -- in those e-mails.