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I girlfriend used an iSkin case with her 3G that had Invisible Shield on it. The case did cause the shield to start to come up on the side.

Not sure if it would be different with an iPhone 4....
 
None.

I'd recommend Power Support film, then any case you like.
 
I have a full IS (front and back) on my iP4, and have gone thru many cases

Griffin Reveal: No, doesn't not work. The back plastic shield sticks to the IS and when you try to take it out of the case, the IS comes right off, completely screwing it up

Griffin Reveal Etch: works ok, for the most part.

Belkin Grip Vue: works fine with the IS (started yellowing, so I returned it)

Best Buy rocketfish tpu case (with back design): might start to peel the IS on the front

Belkin Vue Elipse: the one i'm using now (black/clear): works great
 
I have a full IS (front and back) on my iP4, and have gone thru many cases

Griffin Reveal: No, doesn't not work. The back plastic shield sticks to the IS and when you try to take it out of the case, the IS comes right off, completely screwing it up

Griffin Reveal Etch: works ok, for the most part.

Belkin Grip Vue: works fine with the IS (started yellowing, so I returned it)

Best Buy rocketfish tpu case (with back design): might start to peel the IS on the front

Belkin Vue Elipse: the one i'm using now (black/clear): works great

Thanks for your reply , also i cannot find the Belkin Vue Eclipse in the UK anywhere..Another alternative?
 
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