UPDATE: As of 2010, Ubuntu and Fedora both use iBus and I must say this is a great addition to the IME landscape. This is my new recommendation.
Like many Linux users I’ve never really had much luck with SCIM and Opera but I’m pleased to say I’ve finally found a solution: use UIM.
I’ve seen suggestions to install scim-bridge-qt (most promising), uninstall scim-bridge and start a second instance of SCIM just for Opera. None of these worked for me but I found a suggestion to use UIM instead which did the trick. Here’s what you do:
1. Install Anthy and UIM
2. Add this to the top of your .xinitrc file or equivalent (e.g. .xsession) in your home directory.
export GTK_IM_MODULE="uim"
export QT_IM_MODULE="uim"
uim-xim &
export XMODIFIERS=@im="uim"
uim-toolbar-gtk &The optional last line launches a toolbar which you right-click to edit preferences, for example your key-bindings.
3. Logout and login again, launch Opera and enjoy.
If you have a Japanese keyboard you may need to add this line to the top of the .xinitrc file to make UIM detect the Hankaku-Zenkaku key on the left.
This works at least on Arch Linux. Please leave feedback if it doesn’t work for your distro or if you have other suggestions.
Daniemon's Blog 