You did say nature, not specifically landscape, so that is a bit vague.
I shoot very intimate (tight crop) landscape and nature (where I live there are no sweeping vistas that are easily accessible).
I use a 10D. My main kit, probably used in equal proportions is:
17-40 f/4 L
100 f/2.8 macro (actually use this much more than I contemplated)
70-200 f/4 L with 1.4 TC
For my circumstances I seldom wish for wider but occasionally wish for longer. As such, I just picked up a 300 f/4L (light, but I wouldn't want to lug it around backpacking).
I have a 20D on order. I can't imagine wanting the ultrawide zoom that was just released, but then, like I said, I don't have clean sweeping vistas.
Remember, with the 17-40, you at least get something in the normal focal lenght range. Why would you want to exclude this by only having ultra-wide and tele coverage? 28-35 mm is your bread and butter.
Too bad the 70-200 didn't have true macro capability. I would recommend it and the 17-40 for the 20D. But, as it stands, the 3 lenses sound like a more rounded "nature" kit (at least for me). I don't backpack, so take this with a grain of salt.