Zagg is one of the largest manufacturers of accessories for iDevices and this year was among the first, if not the first, to announce the availability of an InvisibleSHIELD kit for the iPhone 4. The Zagg InvisibleSHIELD kit for the iPhone 4 costs $25, which is the same price as and the iPhone 3G/3GS kit, and can be ordered directly from their website.
I have been using the InvisibleSHIELD kit since I bought my iPhone (August last year) and I can say that it was one of the best choices I made because it protected my phone screen from a lot of scratches/ shots that would certainly have required a change of the screen to remove them.
I don't know how "scratch resistant" the new iPhone 4 is, but if you want to be 100% sure that your phone won't be scratched, then I recommend you buy an InvisibleSHIELD with confidence.
