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):

gtk-theme-name = greybird # Change 'greybird' to whatever Xfce theme you're using

Create ~/.config/xfce4/xinitrc and add the following to it:

#!/bin/sh
GTK2_RC_FILES=$HOME/.config/xfce4/gtkrc
export GTK2_RC_FILES
. /etc/xdg/xfce4/xinitrc

After relogin all Qt apps will use your current Xfce GTK+ theme.