Hey Scott, I know we talked about this particular case before, but Timbuk2 quoted me that their case weighs 2.7ounces. If you were so inclined, you could modify it, and remove the flap...it would then be a slip case. I don't have mine infront of me at this very moment, but I'll verify the weight when I get home. Its got enough padding for backpacking...just pop a cuben drybag over it and the whole thing will be bulletproof.
Edit* Ok, so I weighed the case at 2.9 ounces, that said the same scale weighed my kindle 3 at 8.9 ounces while online testimonials say it should be 8.7 ounces. So timbuk2 is probably right that the case weighs 2.7 ounces, as my scale appears to overshoot by .2 ounces
Looking into this a bit more, it seems like Phil (from the blog Ben links to above) found an even lighter solution... a sub 1.5oz neoprene sleeve off ebay