How to wrap (to group) a list of elements into another element with jquery?

Question

I have the following structure.

Answer 1

var i, num = 3, $ul = $('#ul'), $li = $('#ul > li');
for (i=0;i<$li.length;i+=num) {
    $li.slice(i,i+num).wrapAll('<ul />');
}
$ul.find('> ul').unwrap();

http://jsfiddle.net/Q2bYz/

Answer 2

You can do it with wrapAll:

var a = $('li');
do $(a.slice(0,3)).wrapAll('<ul />');   
while((a = a.slice(3)).length>0)

example: http://jsfiddle.net/niklasvh/mZr4h/

Answer 3

.wrap() api is what you are looking for