http://www.amazon.co.uk/Lowepro-Pri...1?ie=UTF8&s=electronics&qid=1261079397&sr=8-1
That ticks so many of your boxes, I almost think it's a fix!
I have one and it's superb. I can fit a Nikon D60 and 2 lenses, 3 at a squeeze, plus filters etc in the main camera compartment. Then there's the other compartment for all the rest of your walking gear. It's also got a handy zip with an extension handle to be able to take the camera out with the rucksack still on your back. Takes a bit of practice, and only suits right handers really.
Access is through the rear, which sounds odd, but it means you can put your rucksack flat on the dirty ground, get at all your gear, and you put the clean side back on your back again.
Also you can tighten the waist belt, slip off the shoulder straps and spin it to the front to flip it down and access all your gear at waist height.
It's a great bag, I like it a lot.
Further, I've used it for hiking in Scotland. I don't think it's biodegradable, however I can't remember the details exactly but it even has some environmental credentials - made from X% recyclables or something.
Finally, wait and see when Amazon (Amazon proper, not a marketplace seller) drops the price. I got mine for about £40 iirc.
PS Welcome to the site!