According to Wikipedia, Eisenhower deployed the 101st Airborn to Little Rock to integrate Central High because Faubus had used the National Guard to block integration. However, it also states that Eisenhower federalized the National Guard and used it along with the 101st to force integration. So why didn't he just federalize the Arkansas National Guard from the start and not bother with federal soldiers? Was the National Guard's loyalty that dubious or was there some procedural issue that prevented the president from using them?
Eisenhower quotes a telegram from the mayor of Little Rock, Woodrow Mann in his book Waging Peace which states that "the immediate need for federal troops is urgent". (Pp. 170) In Eisenhower: Soldier and President, Stephen Ambrose states (though he cites no source so it may be speculation) that Eisenhower wanted to demonstrate how rapidly the Army could respond to a domestic situation.
It seems possible that after a notable lack of taking one side or another on the issue, Eisenhower wanted to come down hard on one side of the fence and demonstrate commitment through a powerful gesture. Eisenhower had hoped that the issue of desegregation would be handled without his involvement, even past the point where to most people it seems that it should have been obvious that he would need to state a clear position. It may be that calling in the army was a political and PR move as much as a practical one. However, this is merely my own speculation, so if anyone knows more about the matter I will bow to them.