In particular, Apple's bumper cases will not fit the new Verizon version. It's not all bad news, however. The company is already reworking its bumpers to fit the new Verizon iPhone 4. It's unclear when those will be available, or if they already are, but it's a few weeks until the launch.
As far as third-party manufacturers go, you bet there are more than a few who are going to have to rework their cases.