Xfce will never be automatically upgraded to 4.8 in Squeeze (Debian-Stable), because that would not be a "stable" Operating System . . . Imagine you are sitting in your office, rushing to meet a work deadline. Then someone runs in and pulls your desk out from under you; and says "it's time to upgrade to a new version of your desk!"
The new Xfce-4.8 desktop is now in Debian's Testing repros . . . but, it is a major change and hassle to install it.
Because you must first remove/purge the existing Xfce-4.6 before
you can install the new version.
This is very hard to do ... if you are using Xfce?
Trust me on this . . . I did it.