In the contentious campaign leading up to the 2008 presidential elections, Hillary Clinton was considered a front-runner for the Democratic Party. At the time, Barack Obama was still a junior Senator from Illinois, and while his support grew in leaps and bounds he initially had trouble battling the politically experienced former First Lady on the campaign trail. After Obama pulled ahead in delegate votes, Clinton suspended her campaign and endorsed Obama's run. After winning the election, Obama appointed Clinton to the office of Secretary of State.
In the current campaign, it is generally taken as a done-deal that only Obama is running, and that any challenging Democratic candidate will serve only to take votes away from the incumbent. This is more than just a theory; in 1991, Ross Perot successfully entered his name into the Presidential race and managed to achieve almost 19% of the popular vote, an extraordinary number for a third-party candidate. William Clinton's majority win of 43% is often seen as the direct result of Perot's involvement; if Perot had not won his 19%, incumbent George H.W. Bush would likely have won a second term.
Additionally, Hillary Clinton has stated that she is not interested in running again. In a March 2011 interview with CNN's Wolf Blitzer, she said:
...I am doing what I want to do right now and I have no intention or any idea even of running again. I'm going to do the best I can at this job for the next two years.
At that time, she had no intention of running. If she enters the race as a surprise candidate, it is possible and even likely that she will split the vote between her and Obama, and their campaign battles will weaken his position while allowing Republican candidates to focus on their own campaigns instead of battling Obama. For the Democratic Party, it would be a bad idea, unless Obama suspended his own campaign, which is very unlikely.
In summary, superdelegates will cast their votes for Obama because he is, at the moment, the only Democratic candidate. If Hilary Clinton were to enter the race, they would still be best served to support Obama so as not to split the vote.