[slicecom]

I have yet to see any pictures of black iPhones with cracks in the plastic, but MANY white ones seem to be showing up with cracks. They might be there on some black ones, but just be harder to see.

 

[slicecom]

I wonder if this is an issue with iPhones sold in the first few days?

Nope, people have got iPhones within the past few days (many of them replacements for other defective iPhones) that have developed cracks as well. There's lots of posts about the issue in the macrumors.com forums.