I'd consider one of the polycarbonate cases, as they should stop other items from contacting the screen.
This one from mCover is cheap, but not 100% coverage. They also have models for the K2 and DX
This one from Klearkase, seems the same, but four times the price (and only for the K2). Does have a pic of someone spilling a latte on the Kindle in the case ... I don't suggest you do that, as it also seems not to completely cover the kindle.
The Guardian case from MEdge seems to be the best design (and is half retail, right now). Also available
in red and
in cobalt blue.
It's definitely one I'd consider if I carried mine in a backpack a lot. Or in a sandy environment (or around water, if you read at the pool).