Poul, I had this with Gimp, and xmms. Quick fix (so you can use these apps):
Open xmms. ctrl-P brings up preferences. Take note of the fonts listed, they are the problems ones. Browse to select something that works. For the Gimp, edit .gimp/gtkrc in your home directory. I also sought help on the list and got nowhere. Strangely I have two machines, running unstable, but upgraded at different times. One has this problem one hasn't. I consulted our local user list here in Sydney. Only one or two others had the problem, the others, also running ustable couldn't understand. I also happens with rvplayer. I can't work out how I fixed it, but it was similar to the gimp and xmms. I know that ctrl-R bings up the prefs but I think it's a rc file you need to change. LOL Nick