Hmm that case is very interesting. I checked that Amazon store's other items for the Sony Readers and see there's also an "Executive Jacket". Either one looks like a nice fit.
With the case you mentioned do you read it through the plastic or do you take it out for actual reading? I always have mixed feelings about cases of that nature. You get full protection from dust, spills, etc., but you sacrifice some visual quality at the same time or add in the inconvenience of adding/remove the case regularly. Let me know what life with that case is like.