Your whole code can be reduced to this.

/* secsToDHMS()
 * Author Gary Green
 * http://stackoverflow.com/users/63523/gary-green
 */
function secsToDHMS(s)
{
  var x = [86400, 3600, 60, 1],z,i=-1;
  while (z=x[++i])
        x[i] = ("0" + parseInt(s / z,10)).slice(-2),
        s %= z;

    return x.join(':');
};

(function() {
    
    var currentDate      = new Date();
    var endDate          = new Date('March 23, 2012 22:42:30');
    var secsToGo         = (new Date(endDate - currentDate)).getTime() / 1000;
    var countdownElement = document.getElementById('countdown');
    
    var timer = setInterval(function() {
        
        if (--secsToGo < 0) {
            clearInterval(timer);
            return;
        }
       
        countdownElement.innerHTML = secsToDHMS(secsToGo);
        
    }, 1000);
    
})();

Fiddle: http://jsfiddle.net/xg63V/3

Your whole code can be reduced to this.

/* secsToDHMS()
 * Author Gary Green
 * http://stackoverflow.com/users/63523/gary-green
 */
function secsToDHMS(s)
{
  var x = [86400, 3600, 60, 1],z,i=-1;
  while (z=x[++i])
        x[i] = ("0" + parseInt(s / z,10)).slice(-2),
        s %= z;

    return x.join(':');
};

(function() {
    
    var currentDate      = new Date();
    var endDate          = new Date('March 30, 2012 18:10:00');
    var secsToGo         = (new Date(endDate - currentDate)).getTime() / 1000;
    var countdownElement = document.getElementById('countdown');
    
    var timer = setInterval(function() {
        
        if (--secsToGo < 0) {
            clearInterval(timer);
            return;
        }
       
        countdownElement.innerHTML = secsToDHMS(secsToGo);
        
    }, 1000);
    
})();

Fiddle: http://jsfiddle.net/xg63V/4

Compressed version 311 bytes

(function(){var d=(new Date(new Date("March 30, 2012 18:10:00")-new Date)).getTime()/1E3,
f=document.getElementById("countdown"),g=setInterval(function(){if(0>--d)clearInterval(g);
else{for(var a=d,b=[86400,3600,60,1],c,e=-1;c=b[++e];)b[e]=("0"+parseInt(a/c,10)).slice(-2),
a%=c;f.innerHTML=b.join(":")}},1E3)})();​

Your whole code can be reduced to this.

/* secsToDHMS()
 * Author Gary Green
 * http://stackoverflow.com/users/63523/gary-green
 */
function secsToDHMS(s)
{
  var x = [86400, 3600, 60, 1],z,i=-1;
  while (z=x[++i])
        x[i] = ("0" + parseInt(s / z,10)).slice(-2),
        s %= z;

    return x.join(':');
};
 

(function() {
    
    var currentDate      = new Date();
    var endDate          = new Date('March 30, 2012 18:10:00');
    var secsToGo         = (new Date(endDate - currentDate)).getTime() / 1000;
    var countdownElement = document.getElementById('countdown');
    
    var timer = setInterval(function() {
        
        if (! --secsToGo) clearInterval(timer);
       
        countdownElement.innerHTML = secsToDHMS(secsToGo);
        
    }, 1000);
    
})();

​

Fiddle: http://jsfiddle.net/xg63V/2

Your whole code can be reduced to this.

/* secsToDHMS()
 * Author Gary Green
 * http://stackoverflow.com/users/63523/gary-green
 */
function secsToDHMS(s)
{
  var x = [86400, 3600, 60, 1],z,i=-1;
  while (z=x[++i])
        x[i] = ("0" + parseInt(s / z,10)).slice(-2),
        s %= z;

    return x.join(':');
};

(function() {
    
    var currentDate      = new Date();
    var endDate          = new Date('March 23, 2012 22:42:30');
    var secsToGo         = (new Date(endDate - currentDate)).getTime() / 1000;
    var countdownElement = document.getElementById('countdown');
    
    var timer = setInterval(function() {
        
        if (--secsToGo < 0) {
            clearInterval(timer);
            return;
        }
       
        countdownElement.innerHTML = secsToDHMS(secsToGo);
        
    }, 1000);
    
})();

Fiddle: http://jsfiddle.net/xg63V/3