According to the 14th Amendment to the US Constitution, electors are enjoined from voting for someone who is ineligible to take the office of the presidency.
The eligiblity of Barack Obama to take the presidency has not been established, although many ambiguous reports in the media have claimed otherwise or misrepresented the situation.
According to some lawyers, electors voting for ineligible candidates might be committing vote fraud. See this recently filed legal action by former UN Ambassador Allen Keyes et al against Obama et al:
Aiding and abetting fraud in relation to voting is considered a felony and can be punishable up to 3 years in state prison (the length of the prison term differs state to state).