These are the unbreakable panels that I sell in my store. They held up and still work after 3000 miles. While I guess they could break if something hits them at say 80 mph with a sharp point. I personally didn't have any problems. I have walked and even installed panels on decks before with no issues. There is always an exception to this rule.
Of course I think if it would break a solar panel, it would also shred into teardrop possibly.
http://www.outsidesupply.com/index.asp?PageAction=VIEWCATS&Category=13