If you need to get id of selected simple product in configurable product on client side you can do it in many different ways.

Here is simple function how to achieve that with no code modification, new templates or even modules.
Just one Javascript file and layout update.

Product.Config.prototype.getIdOfSelectedProduct = function()
{
     var existingProducts = new Object();
 
     for(var i=this.settings.length-1;i>=0;i--)
     {
         var selected = this.settings[i].options[this.settings[i].selectedIndex];
         if(selected.config)
         {
         	for(var iproducts=0;iproducts<selected.config.products.length;iproducts++)
         	{
         		var usedAsKey = selected.config.products[iproducts]+"";
         		if(existingProducts[usedAsKey]==undefined)
         		{
         			existingProducts[usedAsKey]=1;
         		}
         		else
         		{
         			existingProducts[usedAsKey]=existingProducts[usedAsKey]+1;
         		}
             }
         }
     }
 
     for (var keyValue in existingProducts)
     {
     	for ( var keyValueInner in existingProducts)
         {
         	if(Number(existingProducts[keyValueInner])<Number(existingProducts[keyValue]))
         	{
         		delete existingProducts[keyValueInner];
         	}
         }
     }
 
     var sizeOfExistingProducts=0;
     var currentSimpleProductId = "";
     for ( var keyValue in existingProducts)
     {
     	currentSimpleProductId = keyValue;
     	sizeOfExistingProducts=sizeOfExistingProducts+1
     }
 
     if(sizeOfExistingProducts==1)
     {
    	 alert("Selected product is: "+currentSimpleProductId)
     }
}