the US vice presidency.

Using the online pseudonym "rubico", Mr Kernell - a student whose father is a senior Democrat politician in Tennessee - answered a series of security questions that gave him access to her private inbox, and then shared the details online.

A copy was retained by Wikileaks, the whistle-blowing website currently at the centre of a controversy over leaked US diplomatic cables, and details of her messages were published in several media outlets.

As a result, Ms Palin's family received abusive emails and phone calls. A subsequent FBI investigation led to Mr Kernell's arrest five days later.

Although he was eventually charged with four crimes - including identity theft and fraud - a court in Knoxville, Tennessee, only found him guilty of two lesser counts after a two-week triallast May.

The US Bureau of Prisons (BOP) would not comment on why Judge Phillips' recommendations had not been followed, but said decisions concerning inmates took into account a number of factors.

The BOP is not bound by judicial recommendations, one legal expert said federal sentencing was often "arbitrary".

"The judge can give either incarceration or probation, but if it's incarceration the state gives power to the Bureau of Prisons to determine the nature of incarceration," said Professor Robert Weisberg, director of the criminal justice center at Stanford University in California.

"There is not a general or uniform US rule," he added. "There is huge local variation."

Ms Palin - now seen as a potential presidential candidate in 2012 - has been in the headlines again after the fatal shootings in Arizona that