T5BRICK
macrumors G3
I've had a 3Gs for years. The plastic seems very strong to me. I never even bothered to put it in a case. I think it would take a pretty hard impact to crack it.
The plastic on the 3G and 3Gs was prone to hairline cracks, which just got worse over time. It was much more noticeable on the white models, but it happened on the black ones too.