Yeah, I'd consider the word "Simple" is what best describes Apple. If one size doesn't fit all, they'll work on the "size", but they'll make it "fit all" some how. Such is the case with Verizon's new iPhone 4, which looks a lot like the current AT&T iPhone 4, except for some antenna differences. These differences make the mute switch on the Verizon iPhone be just a millimeter bellow the place where it is on the other variant, so yeah, current bumper cases won't fit nicely on the future device.
But before you worry about this, just do yourself a favor and have a closer look at the bumper case before you buy. It seems Apple already thought of that and has taken care of this by making the mute switch hole on the Bumper case bigger in order for it to fit on both variants. Given it'll be hard for you to see this while buying a case online, our best advice to you is to just stop by an Apple Store and making sure it works on your new iPhone before you buy.
Now if Apple would only figure out a way to not have their precious "protective" Bumper case scratch the hell out of the Stainless Steel band, I'd join you in buying one.