You need each text to have its own input string. You kept changing the same one.

static var underScore : String[];// = "_";

var guessTextStyle : GUIStyle;

var word : String;
private var lastWord : String;

function Start()
{
    UpdateUnderScoreWhenNewWord();
}
function Update()
{
    // I urge you to call UpdateUnderScoreWhenNewWord() only when you need - string          
    // comparison is costly
    if ( word != lastWord )
      UpdateUnderScoreWhenNewWord();
}

// Currently - this clears word!!!!!
function UpdateUnderScoreWhenNewWord()
{
   lastWord = word;
   underScore = new String[word.Length];
   for(var i=0; i<underScore.Length; i++)
   {
      underScore[i] = "_";
   }
}

function OnGUI() {

    textPosY = Screen.height - 50;
    textPosX = 25;
    i = 0;

    for(i = 0; i <= word.length; i++)
    {
       underScore[i] = GUI.TextArea(Rect(textPosX,textPosY,50,50),underScore[i], 1, guessTextStyle);
       textPosX += 75;
    }

}