MacBook Pro sleeps when lid closed?

Hi all, I have a 15" Macbook Pro, circa late 2007 I believe, silver body, silver keys, model A1211. Until now I've never had occasion to plug an external model into the computer. I know that without an external monitor, closing the lid is supposed to cause the computer to sleep. However, when I asked about this issue around the time I bought the computer on these forums, I thought I was told that if an external display is connected, shutting the lid will not cause the computer to shut off. So today for the first time I've plugged in my TV to the computer via a DVI->HDMI cable, but I am finding that closing the computer's lid still causes hibernation. Is there any way around this? A configuration option? I am using OS X 10.6 if it matters.