Every product commercially released requires confidential design to take place. It is normal for designers to work like this.

Even super-secret companies like Apple require videos, adverts, web-pages, photo-shoots etc. The iPad2 probably already has all of these.

Find yourself a good designer. Ask them to keep it quiet. If need be, get them to sign an NDA.

One request. Please release them from the NDA as soon as it is no longer necessary so that the designer can add the work to their portfolio. More than half of the things I have designed have never seen the light of day and yet are still NDA'd. It's very frustrating as it includes some of my best work.