While investigating bug 1086769, I discovered that inkscape's on-canvas text editing does not handle (some?) multi-byte character entry. Here's what I did...
I installed "scim-chinese" and "scim-gtk2-immodule" which gives me an "SCIM" input module (right-click on text-entry widget to select SCIM). Then, you can hit ctrl-space to switch to chinese-char-entry mode, which lets you type a letter (say, "a") and then select (with numbers) which character you want. In the text widgets, this works great. On the canvas, inkscape would do VERY weird things: it would add the "preedit" character to the _end_ of the string instead of at the cursor position, and then when the final char was selected, it would overwrite the char AT the cursor position.
Basically, the SPTextContext is attempting to handle the "preedit" character events itself, but it doesn't do it right. Since "preedit" handling doesn't seem to be _required_ for input to be processed, I've disabled it. This just means that the "predit" char isn't visible until you're done typing it in. What's weird is that other Input Modules (like, say "IPA") will show their preedit letters and inkscape seems to handle it correctly (type "a" "e" with the IPA IM, to see a merged "ae" character). Even with preedit disabled, this works on the canvas.
So... I'm not sure why preediting is there at all, and I've disabled it. Can anyone find any reduced functionality as a result of this? Currently this _fixes_ chinese input (at least with SCIM), so I'd like to leave it like this until I can better understand the IM system.