I've only seen it once but I was logged in.I've been installing DoubleClick for Publishers for the past couple of days. It's supposed to be better than adsense but it's a lot more complicated to set up and get working.
I didn't think it was working at all, so it's weird you saw an ad. It's been really frustrating... The way it's supposed to work is that only unregistered lurkers see ads or the admin see ads. I've only seen one. Did you only see it once?