My current ip5 has a tiny hair and few specs of dust under the lens.
I don't use a case but its a non-issue because it doesn't affect my pics.
I thought the cameras were a sealed unit.
There's a quality control issue with the iPhone 5's for some reason, but the camera design itself shouldn't let more dust in. I had to have my iPhone 5 replaced because of an extremely large amount of dust inside within 2 weeks of owning my phone. My replacement doesn't seem to have any dust accumulating.
I don't know how the poster above was able to have dust cleaned from the lens. The genius said he was going to dust out the inside but then came back out saying he couldn't. The iPhone 5 opens from the screen side so accessing the rear glass is very difficult.