I just got Traditional Chinese input working under Linux. It's actually more straight forward than I thought.
First, install the package scim-chewing. It will pull in all the necessary dependencies.
Next, set up a few environment variables (I put these in my .zshenv file):
export XMODIFIERS="@im=SCIM" export GTK_IM_MODULE="xim"
Finally, launch scim. An icon will appear in the status bar, where you can configure things.
Update: To enable it in gnome, you need to run
im-switch -s scim
and then reboot.
Update 2011-05-11: You probably want to instead use:
im-switch -s scim-bridge
I discovered that scim mode causes problems with rxvt and other C++ applications.