A lot of companies will jump on this. Mozo was just working with MS closely. You will likely see lots of accessories on amazon and other places. You can go with the standard ones. The white ones look really good in person. Go to an MS store and look at the devices. People are saying they are so much better when you hold them yourself.
Well the Sydney Microsoft Store isn't opened yet but I will!
Still this can be a problem (we'll have yet to see) to other countries that doesn't have Microsoft Store though (Converted Nokia ones). Thing is not everybody wants or know that these cases exist online, I came from emerging country where people doesn't do much online transaction as the developed countries do but still have big purchasing power when it comes to phones, which is why Android devices thrived on those countries much better than iPhone, also Windows Phone too thanks to Nokia's effort.
If these cases will be available together with the devices on the same store, it will help the selling point of these new Lumia's. People love personalizing phones, especially something like these shells. Lumia is pretty weak in terms of accessories availability and Microsoft needs to push this, maybe this is easier than app gap issue.