For all-day, take-anywhere protection, the black Case for the iPhone 4/4s from Lifeproof makes a great choice. The case has a full IP-68 rating against water and dust, and is designed according to military specifications. It will guard against shocks and impacts to your phone, and remains fully sealed against minute dust particles. You can even take it underwater during trips to the beach or pool. While offering unsurpassed protection, the case itself only adds a scant 1/16" to your phone, so it won't add much bulk when you're carrying it around.