While the existing answers are on the right track, neither is entirely correct. The plastic cover you're looking at is the shutter - a device that controls the duration for which light is allowed to hit the sensor. The Lumia 1020, like several of its venerable predecessors, has a mechanical shutter (specifically, I believe, a simple leaf shutter), where one or more small plates move to expose and cover the lens assembly. The "crack" that you're seeing, then, is where the two plates meet.
By contrast, most other cameraphones have an electronic shutter, where the same thing is achieved without moving parts, by scanning the sensor either line-by-line (rolling sensor), or in its entirety to a duplicate sensor (global shutter).
The following (computer-generated) image from the Lumia 1020 launch event (full promo video here) shows the shutter opening and letting in light:
This video of a 1020 prototype shows the shutter in action. Specifically, note the following steps:
- As the camera app is launched, the shutter opens so the screen can be used as a viewfinder.
- The lens assembly moves back and forth in order to achieve focus lock.
- Before the exposure begins, the shutter closes momentarily.
- The shutter opens for the duration of the exposure.
- The shutter closes again, and remains closed until the sensor data is fully read.
- The shutter opens again so the user can start taking another picture.