Here's what I've been doing. This is a little different direction, and you might not care for this at all, but it's worth listing here with the more traditional dedicated photo bag approaches.

I arrived at this solution largely because I do most of my photography while hiking, which menas that I've got other stuff to carry (besides photo gear) and I want fast access to my camera and lenses. I use a regular (non-photo) backpack on my back to carry general gear, plus photo items I use less often such as a strobe, cleaning kit, extra batteries, and so on. On my chest, I clip on a LowePro TLZ (available in several sizes) and LowePro Lens Case (I'm using the 2S, which I don't see listed on Amazon anymore). The LowePro lens cases use a sliplock attachment that hooks it securely to the TLZ.

Front view, showing TLZ and lens case:

In this photo, of course, I wasn't photographing wildlife, but I think you get the idea.

Advantages of this approach:

• I can use any backpack I want on my back. In this photo, I've got a small day-pack, but I can also use a large backpacking outfit in the same way. When choosing a backpack, it's helpful to get one with a sternum strap.
• I can use a backpack with a hydration bladder. For some reason, photo backpack manufacturers seem reluctant to include spots in their packs for hydration bladders. Go figure...
• I have immediate access to my camera and lenses (I can carry additional lenses in back if I wish). When hiking, it's helpful to have my camera handy if I do see any wildlife, and when I'm hiking with others, they usually don't want to wait for me to dismount my pack, unload my camera, take photos, and then put everything away again.
• The TLZ top open away from my chest, which helps with access to CF cards and permits lens change on the fly.
• Both the TLZ and lens case can be sized precisely for my gear.
• If I don't want to use a backpack at all, I can clip a shoulder strap onto the TLZ and carry that (with or without lens case) on my shoulder.

I haven't found this solution to be perfect, though. Disadvantages:

• It's little bit of a pain to put on and take off -- probably an extra minute or so, versus just mounting a pack.
• This solution by itself doesn't help with tripod access. There will obviously be plenty of places on your backpack to clip on a tripod, though, and if I'm hiking with someone, I usually just ask someone to grab it off my back if I need it.
• In the combination I'm using, the TLZ bag has an AW cover, but the lens case doesn't. You might be able to find a lens case with cover, though.
• I'm still experimenting with ways to clip the TLZ to my chest without the lens case causing it to sag to that side. You can see that tilt in the photo.