What is X3 4-pin header conneaction near USB for?

My name is Ric and I got my first Arduino Diecimila tonight.

Room: Hello, Ric.

I'm addicted already (form reading online) but have a question: On this board it looks like there's a place for a 0.1" paced 4 pin header near the USB circuit that's just soldered over.

Anyone know what that heard would be for?

Thanks! Ric

Please,

read this: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1184164578

and this: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1180915644/7

Sorry! I thought I'd already searched the forum. My bad.

Thanks!