It seems like whatever the problem was, it's gone now. An up-to-date testing system with ibus runs smoothly.
For some reason, after updating today I'm having a hard time entering input in various gnome applications, whereas non-gnome applications work fine.
E.g. I can't type certain letters, and in general it's difficult to provide any inpurt, in gnome-terminal and gnome-session-properties. lxterm and guake work completely normal though.
No sure what is causing this, but it's happening on my Thinkpad SL410 as well as on my Optiplex 910. The only thing they have in common is the operating system (up-to-date debian testing 64 bit).
Don't know enough about it to file a bug report.
Edit: the problem is present in Evolution as well. Still present as of 28/11/2011. Problem exists on a home-built six-core AMD with 1 GB graphics card.
Edit 2: It sounds a bit like this, which was reported in June (!).
It seems to be related to iBus/SCIM
Edit 3: Evolution is also affected, as is gEdit. Turning off iBus 'solves' the problem. Now, why does it affect the standard gnome applications, but not other packages?
At any rate, this bug has given me the incentive to leave gnome-terminal for guake, and to give alpine and thunderbird a try instead of evolution.
sudo apt-get install ibus-gtk3
sudo apt-get autoremove ibus-gtk
iBus still works. But unfortunately evolution is still way too slow to be useful.