Although I am against her on the Court, I am sorry to disappoint you concerning Citizen's United [or any other case she's argued since March 2009].
The fact is that she is the Solicitor General [aka the Government's Lawyer]. She is the one that argues on behalf of the government - its her job.
She doesn't select the cases that she is assigned and she doesn't select the position that the government takes in those cases. She is told what the position is [by Obama and Company] - then she has to craft her arguments around it.
The job really sucks - if you accept the nomination [and are confirmed], sometimes you have to argue positions in front of SCOTUS that you really don't believe in.
I was not very clear in my last post, I think - I am not saying that Kagan disagreed with the position she had to take in the Citizens United case. But, there are times when she probably would - such as the administration’s position defending the Defense Of Marriage act. If it ever gets to SCOTUS and she is the Solicitor General - she would have to bite her tongue and argue for it ...