Although the iPhone offerings from other carriers and Verizon may look the same, they're just different enough that you'll need to be careful when buying a case or similar accessories.

The big news of the day, of course, is that US-based Verizon Wireless customers will soon--finally--be able to get the iPhone on their carrier of choice. And during Verizon's big media event announcing the arrival of the iPhone, both Verizon and Apple emphasized that this was the same iPhone we've all come to know so intimately--thanks, in part, to controversies over the iPhone 4's antenna design and back surface.

But it turns out there are a couple minor physical differences, and those differences are just enough to affect the Verizon iPhone's compatibility with some of the thousands of iPhone 4 cases out there.

It appears the Verizon iPhone uses a slightly different antenna design that repositions the gaps between the phone's multiple external antennas (found in the metal ring around the phone's outer edge). Along with this change, the phone's Ring/Silent switch has been moved ever-so-slightly closer to the top of the phone.

While the difference between the switch positions may seem small, it's enough that a protective iPhone case made for the original iPhone 4 may not work with the Verizon iPhone 4. Specifically, if a case covers the edges of the phone, providing precision openings for accessing the volume buttons and Ring/Silent switch, there's a good chance that the switch opening will be in the wrong location--in other words, the case will block access to the Ring/Silent switch.

Indeed, Verizon's iPhone 4 Frequently Asked Questions page explicitly notes possible incompatibilities:

If I have an iPhone 4 on another wireless carrier and switch to an iPhone 4 on Verizon Wireless, will all of my accessories be compatible?

Most accessories should work, however you may find that cases and covers could be different depending on the product manufacturer.

It's likely this change will also affect other accessories that cover the iPhone, such as battery packs and stands that wrap around the phone's edges, as well as iPhone skins and protective films.

For many iPhone-accessory vendors, this means retooling some--or even all--of their existing cases for compatibility with the Verizon iPhone. As a Case-Mate representative told Macworld, "We're aware of the difference and have had to adjust our molds to accommodate the small change. The nine new cases we will have available in the next few weeks will all work specifically with the Verizon iPhone."