As you may have noticed, Qt apps no longer use GTK+ theme in Xubuntu 11.10. For me this became particularly noticeable in Psi+. The problem is caused by QGtkStyle, which cannot find GTK+ theme since Xfce stores it in an unusual location (not sure why 11.04 could find it though).
To fix the problem add the following to
~/.config/xfce4/gtkrc (create the file if it does not exist):
~/.config/xfce4/xinitrc and add the following to it:
1 2 3 4
After relogin all Qt apps will use your current Xfce GTK+ theme.