I'm sorry to tell you that but that's not very possible (see Step 17 of this teardown: [guide|3562] : '''For those of you who are wondering, there's no way the iPhone 4's rear camera can be installed in the Touch without some extreme hackery.''').
That's why an iPhone is ~$390 more expensive, obviously.
Hello,
I'm sorry to tell you that but that's not very possible (see Step 17 of this teardown: [guide|3562] : '''For those of you who are wondering, there's no way the iPhone 4's rear camera can be installed in the Touch without some extreme hackery.''').