Home buttons that could be a parts for the next generation iPhone 5 have started turning up online. They are available in black and white from TVC-Mall and TrueSupplier and appear to have a different rubber gasket than the current iPhone 4S design.
It seems as if part leaks have become an honored tradition when it comes to iPhone launches. We see similar leaks when it comes to iPad and iPod touch as well. This also isn't the first time that we've seen a part supplier leak parts ahead of a launch or make them publicly available for sale.
From the pictures on their respective sites in comparison to an iPhone 4S or iPhone 4 home button, the only thing that appears to have changed is the rubber gasket and mold they're using. It doesn't give us much of a clue as to what we can expect in the way of form factor or size change. The square portion would most likely be seated completely below the frame assembly and be an unnoticeable change to users. My guess would be that Apple is just adding a bit more stability.
Apple did something similar when they jumped from the iPhone 4S to the iPhone 4. While the button remained the same, they added a rubber gasket around it which seems to alleviate a lot of the issues users were experiencing when it came to the iPhone 4 home button. The iPhone 4 home button is basically "taped" on to a flimsy sensor cable that wears down over time. The iPhone 4S home button is a bit more sturdy due to the rubber gasket and somewhat improved contact.
I still think it's too early for us to see genuine part leaks for the next generation iPhone. Any little detail could change when it comes to the actual production model. Especially something such as a home button that has no moving parts involved. They could just be changing out the rubber gasket for a more solid button assembly. For now, I'd say it's too early to tell whether or not this could end up in a production unit.