This interesting article at U.S. Politics blog explains the law on which presidential candidates get Secret Service protection. It is significant that the candidate must be seeking the nomination of a party that got at least 10% of the vote in the last election. That standard is lower than the current 15% standard used by the Commission on Presidential Debates.
The article says no non-incumbent presidential candidate in the 2012 election received protection until February 2012. By contrast, in the 2008 election, candidate Barack Obama first started receiving protection on May 3, 2007, according to this New York Times story.