I just went ahead and bought a Knomo Envelope Case (Brown) for my 13" MBA. I bought it from Knomo's eBay outlet store for £21.00 (incl. postage).
The reviews seem good. The only negative seems to be the back of the poppers on the inside of the case may potentially scratch the MBA. I thought I would stick some felt to the back of them to prevent this.
It does say it will have some cosmetic defects, but since it's meant to be genuine leather, this should add to the "character" of it. Since I'm paying less than half price of what it should retail for, I can live with that. I they're excessive I can return it per their guarantee.
There are a few more left here.
My new case just arrived. Cosmetically it is more or less perfect. It came with all the original tags and card inserts, etc. The popper fastenings won't be an issue either since the velvet-like lining covers the back of the poppers on the inside of the case. The case feels quite substantial too - it certainly doesn't feel lightweight or flimsy. Overall, I'm really pleased with it. It's a premium case at a bargain price!