From the announcement post:
Rather than make the typical clip-on case, or carry pouch to put your phone into, the Nokia Lumia 620 protective shell (CC-3061) replaces the existing back cover.
With IP54 certification, the protective shell has been certified to improve protection against dust and splashes of water. For anyone needing or just wanting extra shield for their phone, this replaceable shell is a one to have.
The protective shell improves the dust and splash protection. The gaps between the camera window and the shell as well as the display and the shell are better sealed, and the mesh that covers the loudspeaker is denser, making it harder for water or dust to get in.
Just like the other Nokia Lumia 620 replaceable shells, the protective shell has been cast using a dual-shot process. The interior shell is created using a polycarbonate that’s fairly rigid, and the exterior is made of TPSiV, which is more flexible, rubber-like material.
The combination of both of these materials provides a more flexible plastic; one that’s solid enough to take the forces that your phone may be subjected to, but soft and elastic enough to provide some give when it’s needed.
When the protective shell is attached to the Lumia 620, the rubber-like material wraps around the front slightly, more so than the non-protective shell. This raised ‘lip’ protects the screen more if phone is accidentally dropped.
While the CC-3061 obviously doesn't make the Lumia 620 robust to the same degree as specialist handsets like the Motorola DEFY range, it’s still handy for those who enjoy an active lifestyle where "working up a sweat, getting caught up in a rain or dropping the phone can be a regular occurrence".
The CC-3061 will be only (so far) available in grey with a yellow interior, from the end of this month, depending on market. The recommended retail price is around 25 EUR.