Okay, before you read too far into this, know that Apple won’t replace your iPhone 4 if you break it yourself, so don’t even think about “accidentally” dropping it. They won’t pity you. However, if it stops working somehow, Apple will replace it. The cool part? They may replace it with an iPhone 4S. Again, that’s a MAY.
Why? It seems that Black iPhone 4 units are in low supply, and that Apple is simply telling stores to replace defective ones with the black iPhone 4S to appease customers. Not a bad deal. Of course, you have to live near an Apple Store, and you have to hope that they are low on iPhone 4 inventory. If they are, you may get a free upgrade.