You can nade down hills like on Danger Canyon. The places you cannot nade down are bridges, ledges and the large bases:
- bridges and ledges on Danger Canyon
- cliffs on Blood Gulch (but bases and hills are ok)
- top of bases on Sidewinder, cliffs on Sidewinder, but you may nade a hog on the ramps if you are walking down the ramps
- from upper level on Gephy down to the lower level (ok if you're passenger in a hog though)
- bridge, ledges and Ice Fields (and bases, but you shouldn't be there anyway)
- bases on Infinity (not allowed on bridges)
- top of bases on Timberland into the spawn area, or from the top of the big jump down to the bottom - once you're one your way down I think it's fine, and it's ok on the smaller side hills
- Death Island - if you spawn on a base, you can throw one nade down if necessary (otherwise by the time you jump down they can be gone with all the hogs), otherwise not allowed
And yes, you cannot throw nades over hills, around corners and so forth (happens a lot on Danger Canyon and Timberland) in anticipation that a hog might come along and trip over one. You have to be able to see your target. It's not cool to just chuck nade after nade while walking in hopes it hits something that might come along.
edit: oops jane beat me to it!