In Debian Sarge, SCIM Just Worked. Guess it was a bug, really.
Now, progress makes you free, y'know. Today, in Debian Etch, you also have to — explicitly and manually — let apps know it's there. Progress is a great thing, now isn't it?
So, make sure that when an app is starting, it has a GTK_IM_MODULE="scim" in its environment.
At least, with Iceape (elsewhere known as Mozilla Suite) it's relatively simple. Just drop the line to .mozilla/iceaperc. 没事、哈哈。