I'm sure they have, but I'm equally sure that they still consider themselves "rock". Then again, my source for that is from an interview on their nigh twenty year old Reunion DVD, so maybe they've just given up and said "Yeah, fine, we're metal now."
Either way, I've always been the oddball who didn't think Zeppelin were "metal". Really, I'm not a Zep fan at all. They were hugely influential and all, and certainly influenced plenty of metal and other music, but they just never seemed metal themselves to me. Then again, most people don't put much stock in my opinions, so who cares what I think?