Upgrading to XFCE 4.6 issues

[Marc E. <santusmarc@users.sourceforge.net>]

The following packages have been kept back:
  libxfcegui4-4 thunar-data xfce4 xfce4-panel xfce4-session xfce4- 
utils xfdesktop4-data xfprint4 xfwm4 xfwm4-themes
0 upgraded, 0 newly installed, 0 to remove and 10 not upgraded.

I've noticed that XFCE 4.6 has hit lenny backports, how are we  
supposed to upgrade to it?

Thanks.

Marc