I wrapped my 3GS in an Otterbox Defender case, and my 4S in a Griffin one - both had a plastic shell with a thick rubber outer casing, so damage isn't an issue.
Does mean that I need to reassess my contract because if I'm keeping my 4S, there's no point in paying a monthly charge. I am in need of a PAYG sim but one with either Orange or Vodafone as they're the only two that have a decent signal at work.