Cross browser "jump to"/"scroll" textarea

You can stop wrapping with the wrap attribute. It is not part of HTML 4, but most browsers support it.
You can compute the height of a line by dividing the height of the area by its number of rows.

<script type="text/javascript" language="JavaScript">
function Jump(line)
{
  var ta = document.getElementById("TextArea");
  var lineHeight = ta.clientHeight / ta.rows;
  var jump = (line - 1) * lineHeight;
  ta.scrollTop = jump;
}
</script>

<textarea name="TextArea" id="TextArea" 
  rows="40" cols="80" title="Paste text here"
  wrap="off"></textarea>
<input type="button" onclick="Jump(98)" title="Go!" value="Jump"/>

Tested OK in FF3 and IE6.

If you use .scrollHeight instead of .clientHeight, it will work properly for textareas that are shown with a limited height and a scrollbar:

<script type="text/javascript" language="JavaScript">
function Jump(line)
{
  var ta = document.getElementById("TextArea");
  var lineHeight = ta.scrollHeight / ta.rows;
  var jump = (line - 1) * lineHeight;
  ta.scrollTop = jump;
}
</script>

<textarea name="TextArea" id="TextArea" 
  rows="40" cols="80" title="Paste text here"
  wrap="off"></textarea>
<input type="button" onclick="Jump(98)" title="Go!" value="Jump"/>