I needed to use SCIM for a number of applications on Jaunty, including Firefox and aMSN. The problem is with apps that weren't installed from Ubuntu repositories. The supplied Firefox 3.0 works fine, but not 3.5 or 3.6 I downloaded from Mozilla. There's no Ubuntu version of aMSN for that matter. The problem lies with QT it seems, which SCIM needs to run with and be compatible with the QT used to compile the target apps.
I solved the aMSN piece with help from this Ubuntu community documentation, but Firefox just wouldn't work. Eventually I installed FF 3.5 and 3.6 from Launchpad repositories, and that brought those apps in line with SCIM. BTW, that also solves my Java plug-in issue with them.