This is not something that SVG 1.1 supports. SVG 1.2 does have the textArea element, with automatic word wrapping, but it's not implemented in all browsers. SVG 2 does not plan on implementing textArea, but it does have auto-wrapped text.

However, given that you already know where your linebreaks should occur, you can break your text into multiple <tspan>s, each with x="0" and dy="1.4em" to simulate actual lines of text. For example:

<g transform="translate(123 456)"><!-- replace with your target upper left corner coordinates -->
  <text x="0" y="0">
    <tspan x="0" dy="1.2em">very long text</tspan>
    <tspan x="0" dy="1.2em">I would like to linebreak</tspan>
  </text>
</g>

Of course, since you want to do that from JavaScript, you'll have to manually create and insert each element into the DOM.

I suppese you alredy managed to solve it, but if someone is looking for similar solution then this worked for me:

 g.append('svg:text')
  .attr('x', 0)
  .attr('y', 30)
  .attr('class', 'id')
  .append('svg:tspan')
  .attr('x', 0)
  .attr('dy', 5)
  .text(function(d) { return d.name; })
  .append('svg:tspan')
  .attr('x', 0)
  .attr('dy', 20)
  .text(function(d) { return d.sname; })
  .append('svg:tspan')
  .attr('x', 0)
  .attr('dy', 20)
  .text(function(d) { return d.idcode; })

There are 3 lines separated with linebreak.

With the tspan solution, let's say you don't know in advance where to put your line breaks: you can use this nice function, that I found here: http://bl.ocks.org/mbostock/7555321

That automatically does line breaks for long text svg for a given width in pixel.

function wrap(text, width) {
  text.each(function() {
    var text = d3.select(this),
        words = text.text().split(/\s+/).reverse(),
        word,
        line = [],
        lineNumber = 0,
        lineHeight = 1.1, // ems
        y = text.attr("y"),
        dy = parseFloat(text.attr("dy")) || 0,
        tspan = text.text(null).append("tspan").attr("x", 0).attr("y", y).attr("dy", dy + "em");
    while (word = words.pop()) {
      line.push(word);
      tspan.text(line.join(" "));
      if (tspan.node().getComputedTextLength() > width) {
        line.pop();
        tspan.text(line.join(" "));
        line = [word];
        tspan = text.append("tspan").attr("x", 0).attr("y", y).attr("dy", ++lineNumber * lineHeight + dy + "em").text(word);
      }
    }
  });
}

I think this does what you want:

function ShowTooltip(evt, mouseovertext){
    // Make tooltip text        
    var tooltip_text = tt.childNodes.item(1);
    var words = mouseovertext.split("\\\n");
    var max_length = 0;

    for (var i=0; i<3; i++){
        tooltip_text.childNodes.item(i).firstChild.data = i<words.length ?  words[i] : " ";
        length = tooltip_text.childNodes.item(i).getComputedTextLength();
        if (length > max_length) {max_length = length;}
    }

    var x = evt.clientX + 14 + max_length/2;
    var y = evt.clientY + 29;
    tt.setAttributeNS(null,"transform", "translate(" + x + " " + y + ")")

    // Make tooltip background
    bg.setAttributeNS(null,"width", max_length+15);
    bg.setAttributeNS(null,"height", words.length*15+6);
    bg.setAttributeNS(null,"x",evt.clientX+8);
    bg.setAttributeNS(null,"y",evt.clientY+14);

    // Show everything
    tt.setAttributeNS(null,"visibility","visible");
    bg.setAttributeNS(null,"visibility","visible");
}

It splits the text on \\\n and for each puts each fragment in a tspan. Then it calculates the size of the box required based on the longest length of text and the number of lines. You will also need to change the tooltip text element to contain three tspans:

<g id="tooltip" visibility="hidden">
    <text><tspan>x</tspan><tspan x="0" dy="15">x</tspan><tspan x="0" dy="15">x</tspan></text>
</g>

This assumes that you never have more than three lines. If you want more than three lines you can add more tspans and increase the length of the for loop.

use HTML instead of javascript

limitation: the SVG renderer must support HTML rendering

for example, inkscape cannot render such SVG files

svg foreignObject to embed html

I have adapted a bit the solution by @steco, switching the dependency from d3 to jquery and adding the height of the text element as parameter

function wrap(text, width, height) {
  text.each(function(idx,elem) {
    var text = $(elem);
    text.attr("dy",height);
        var words = text.text().split(/\s+/).reverse(),
        word,
        line = [],
        lineNumber = 0,
        lineHeight = 1.1, // ems
        y = text.attr("y"),
        dy = parseFloat( text.attr("dy") ),
        tspan = text.text(null).append("tspan").attr("x", 0).attr("y", y).attr("dy", dy + "em");
    while (word = words.pop()) {
      line.push(word);
      tspan.text(line.join(" "));
      if (elem.getComputedTextLength() > width) {
        line.pop();
        tspan.text(line.join(" "));
        line = [word];
        tspan = text.append("tspan").attr("x", 0).attr("y", y).attr("dy", ++lineNumber * lineHeight + dy + "em").text(word);
      }
    }
  });
}