Edit: for the most up-to-date solution, see my answer here: http://askubuntu.com/a/363400/18665
Solution:
After much trial and error, I was able to get WebEx desktop sharing working in 64-bit Ubuntu just by installing a few packages (I confirmed this works on Ubuntu 14.04 and 12.04, but I imagine it would work on other versions as well):
Remove unnecessary packages that cause conflicts:sudo apt-get -y remove icedtea-7-plugin:i386...
