As you can see in this picture by Engadget, the Verizon iPhone compared to the AT&T iPhone has a slightly different design. The Verizon iPhone 4 (top) has a modified antenna which puts that black line on the left side rather than on the top next to the headphone jack. This pushes the buttons down just a bit, but that tiny difference is likely enough to require new cases made specifically for the iPhone on Verizon. Buyer beware: if you are in the market for a Verizon iPhone 4, hold off on buying a case for now.

Update (3:15p ET): An Apple spokesperson has told 9to5Mac that Apple will be updating their own Bumper case to support the tweaked design of the iPhone 4 on Verizon. The AT&T version will obviously still be available for purchase as well.