If you can't find a way of doing it with the templates you can always do it by adding a databinding to a property which returns different data depending on whether it's called for the first time or not. Something like:

aspx:

.... <div class='normalclass <%#MyFlag ? String.Empty : "my_extra_class" %>'>...

codebehind:

protected bool MyFlag
{
get
{
bool oldFlag = _myFlag; // private field
_myFlag = true; // Flag that it's been hit
return oldFlag; // Returns false first time, then true for all additional times
}
}

It's probably a good thing to reset the flag in the OnDataBinding event or similar so you know it's state when databinding starts.

You could also check the Container.ItemIndex.