I also had this issue on Snow Leopard about creating an icns file containing 1024x1024 image.
I have been able to generate one using png2icns utility. It comes with libicns and is fairly simple to install.
You will first need to download Jasper at : http://www.ece.uvic.ca/~frodo/jasper/
Also you need libpng. I found it on Softpedia.
Build first jasper, libpng then libicns. To do so , for each of them I just add to cd into the directories where they are unpacked and , from the command tool type the commands :
./configure;
make;
sudo make install;
Then should should have png2icns available on the command line.
To use it, just cd to the directory containing your images and type :
png2icns iconfile.icns image1.png image2.png......