Not a noticeably different one from the outside, some people even think the White Ip4 coming 30th July will have a built in insulator! Apple isn't going to tell anyone of any hardware changes as it doesn't want to swap out 3M ip4s!
I think you should read your post again. "Some people are saying" and "Apple isn't going to tell anyone". So where are these people getting their information from?
After the 30th, that's the deadline for free cases, not the start a new version of the iPhone 4. From the 30th, Apple expect you to know about the problem, so you either buy a case with the phone if you're going to use it left handed in a low signal area or switch hands when needed.