What's the best protective shield for my 3GS?

The reason for no definite answer is that it matters what YOU are looking for in a case or protective screen cover...

To answer your shield question...the shield product itself will not damage the phone and that coating is not to prevent scratches...it's to help prevent fingerprints...since you plan to put a screen cover on then that coating doesn't really matter...HOWEVER, the way that the shield can harm the phone is IF it's not properly installed by YOU...since the design of that product calls for a liquid solution agent to be used to adhere the cover to the case of the phone, others have gone overboard with that liquid and then ended up with water damaged phones due to the faulty install...If you install properly then it's not an issue

Personally I used a full shield cover on a previous phone and hated it...so again it's all up to your taste...

What I currently have now is the simple Apple sold Anti-Glare screen cover you can buy two to a pack for 15.00 at Apple store...I have used since first Gen iPhone and loved it...

I have a Case-Mate leather hard plastic case but am not thrilled with it since the edges are raised around the edges of the screen and sometimes prevent me from hitting a button right up against the screen edge...so I don't use it much...

Other covers made by Case-Mate like the Barely There cover don't have that problem...so you really need to shop around and see what case you like and what you dislike...

Cases specifically sold to be compatible with 3GS will not interfere with either the proximity or any other sensor on the phone face...

get an iskin

Hi, if you are active and want something to really protect your investment I would recommend an Otterbox Defender. I have one and appreciate having it as I have banged and dropped my phone with no effects. Makes the phone kinda big and clunky but that's better than broken to me. Hope ya find something...

I personally don't believe in putting a screen protector on an iPhone. Unlike most other phones (and Palms) the screen is glass, and will not scratch in normal use. (Meaning don't put it in your pocket with your keys without a case that covers the screen, which you should have protector or no). Further, most screen protectors will not protect the phone from scratches; anything that can scratch the glass will do so right through the protector. The 3GS has an amazing coating on it that truly does prevent fingerprints and smudges. Putting a protector over this coating defeats it and means that you WILL get fingerprints and smudges on the screen.