Dropping mobile electronics is a fact of life. If it's something that you carry around with you and handle regularly, it's going to get dropped. You never mean to drop it, but sometimes those fingers of yours are mysteriously and momentarily greased with butter and gravity takes over.
The iPhone 4S shares the same glass front and back design as its predecessor. While this makes for shiny, glossy and relatively scratch proof (compared to plastic) surfaces that are quite radio transparent, they're prone to shattering. Even with the antenna problem fixed in the new phone, owners of the iPhone 4S will be much safer with a case – or the new AppleCare+ plan, which covers accidental damage.
Check out this drop test from third-party warranty provider SquareTrade, which compares the drop survival of an iPhone 4S and Samsung Galaxy S II.