hehe, you have to make a setCurrentActiveAndHideLast function 
Here the whole script (untested)
var current = 0, //start at 0 (first)
last, // define for the last
timebetween = 5000, //time between
tweets = $('.tweet_list li'), //jquery object with all tweets
count = tweets.length, //tweet count
//intervall
interval = setInterval(function(){
//call action
setCurrentActiveAndHideLast(current);
//action is called set the last to the current
last = current;
//if current reach the tweetcount start with the first one again
(count>=current)?current++:current=0;
},timebetween);
//hide all tweetd
tweets.hide();
function setCurrentActiveAndHideLast(current){
//show the current
tweets.eq(current).slideDown();
//and hide the last if set
if(last != null)tweets.eq(last).slideUp();
}