I'm not sure I'm getting you right, but if I am, I'd first create a new element:
var newEle = $(str);
Then prepend it, hide and animate
$('#news-ul').prepend(newEle);
$(newEle).hide().slideDown();
Hope this helps.
Edit, more code:
Take a look at http://api.jquery.com/category/effects/ If I were to recreate the twitter effect it would probably be:
$('#news-ul').prepend(newEle);
$(newEle).fadeTo(0,0.01,function(){
$(this).hide().slideDown('normal',function(){
$(this).fadeIn();
});
});
A quick description, first we add the element as before, then fade it out to 1% opacity in 0 seconds (at 0 opacity it has no height), then, when we know its faded, we issue a callback, hiding the element completely, issue the slidedown with a normal speed, when done sliding, we issue another callback fading it in.
Warning: The above code is untested, use at your own risk ^^