No, putting your phone in your back pocket has always been crazy. Some people do it, but it doesn't make it any less crazy. The only that's changed recently is that people have been discussing the practice of putting one's phone in a back pocket as a result of this story, to which the only rational response is, "that's crazy!" :) This is not a usage pattern that any handset maker tests for or designs their devices to deal with. If it works for you, great, but that's more a testament to either your relatively light weight or the sturdiness of the iPhone 4. It is most definitely not something the people who make smartphones assume their users are going to do.