Will my existing AT&T case / bumper fit the Verizon iPhone 4?

If you already own an iPhone 4 case it might fit the Verizon iPhone 4. This depends on the design of the case. Anything form-fitting with separate cutouts for the volume buttons and ring/silent switch will NOT fit the Verizon iPhone. Verizon's model is different, with the ring/silent switch moved down closer to the volume buttons to accommodate changes in the antenna.

This means that cases like the Apple iPhone 4 Bumper sold to AT&T customers will not fit the Verizon iPhone. Manufacturers are now producing special form-fitting cases that will fit the Verizon iPhone.