I've had my 3GS in an Otter Box and it saved my precious countless times. Granted it is the ugliest thing you ever layed eyes on. It also adds so much more bulk. However, I've had many other cases and it's second to none in protecting my precious!
I'll have my iPhone 4 in just about 17 hours from now. But it won't leave the house until I have her tucked away in a new Otter Box. I'm on the e mail notification list with them as to when they will be available. I'll post a note when I get notice. Until then my trusty 3GS will stay activated. By the way it's in prestine condition and up for sale! LOL. Teasing... I'll keep it in my toy box!
I also had mine in an Otterbox and loved it. Nothing will protect a phone like an Otterbox. But I sold my 3GS along with my Otterbox getting ready for the new iPhone, so I'm in a jam. I will just have to baby it for a while. I'm also on there mailing list.