The great thing about .append() and .appendTo() is that they will move existing DOM elements, not copy them, as you might expect:
$('ul').children('li').appendTo('ul:first'); // move all LIs to the first UL $('ul').not(':first').remove(); // delete the extra ULs
Set the 'ul' selector as needed; I recommend using a generic class instead of a generic tag switcher.
http://jsfiddle.net/j76Lu/
Slightly optimized (thanks, adeneo ):
$('ul').not(':first').remove().children('li').appendTo('ul:first');
http://jsfiddle.net/j76Lu/1/
or even better:
$('ul:gt(0)').remove().children('li').appendTo('ul:eq(0)');
http://jsfiddle.net/j76Lu/2/
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