I used a Meguiars product called PlastX Clear Plastic Polish and a special Meguiars microfiber cloth on three dust covers last week. Recommended by another Beoworld member. I'ts a really great product for minor scratches. It's blue when applied and then changes into a clear substance.
It contains mineral oils that makes the dust cover look brand new. I've also had great results with this stuff on the plastic displays of myBeosystem 7000 units. It takes of the very very top layer of the display without scratching at all. The displays now have that B&O shop shine again. The effect is even greater when you turn on a unit and the led's light up!
Great stuff, bought it at a car tuning shop. You can also get it online.