I keep my iPhone in a case and have "iPhone OCD", but with good reason.
I bought my iPhone 3G in September 09 for £342 on O2 Pay & Go, so I'm not in the position of having a contract that will expire and if I've honoured it then I get a good upgrade to the newest iPhone as and when it comes out.
This iPhone is the iPhone I intend to own for the next 18 months, maybe longer ... who knows?
Since getting my iPhone replaced in December due to a cracked headphone socket, I have used the replacement in a Contour Flick case that I got for a bargain £5 at the HMV Christmas sale. A lot of people have moaned that this case scratches the phone, but I've removed my iPhone from it and put it back in after cleaning and the bezel and back are perfect ... and trust me, I've checked under numerous light conditions!
I used to own an Invisible Shield on the original iPhone I bought, so I still have strips of it left to use on the bottom of the bezel where the Flick doesn't cover.
Add to this the screen protector, and I'm good to go. The Flick doesn't add much bulk to the iPhone, and the feeling you get when taking it out for a clean or to simply use around the house, and seeing how pristine it is, is superb.
If I was on a contract and knew I would be able to upgrade at a lesser cost compared to Pay & Go, I wouldn't care ... but this is MY iPhone, not one taken out on debt and paid up over 12/18/24 months. Sue me if I want to keep it looking in as perfect condition as possible.