Assuming this:
The accepted answer by Bobince is a short, portable solution. If you need to not only append SVG but also manipulate it, you could try the JavaScript library "Pablo" (I wrote it). It will feel familiar to jQuery users.
Your code example would then look like:
$(document).ready(function(){
Pablo("svg").append(' ');
});
You can also create SVG elements on the fly, without specifying markup:
var circle = Pablo.circle({
cx:100,
cy:50,
r:40
}).appendTo('svg');